fixing default for THEME_NAME
fixing default for THEME_NAME

If THEME_NAME is set to an empty string it will cause a sass error due
to this mako code:

```
% if env.get('THEME_NAME') is not None:
  // import theme's Sass overrides
  @import '${env.get('THEME_NAME')}';
% endif
```

michaelsteiner19/open-edx-configuration,bugcy013/edx_infra,chudaol/configuration,vasyarv/configuration,ovnicraft/evex-configuration,appsembler/configuration,bugcy013/edx_infra,gashac03/ubuntu,LearnEra/LearnEra-Configuration,chudaol/configuration,armaan/edx-configuration,edx/configuration,hastexo/edx-configuration,hastexo/edx-configuration-old,4eek/configuration,armaan/edx-configuration,IndonesiaX/configuration,hastexo/edx-configuration,rue89-tech/configuration,jp-bpl/configuration,Stanford-Online/configuration,hks-epod/configuration,glengal/configuration,cecep-edu/configuration,open-craft/configuration,kencung/configuration,usernamenumber/configuration,hmcmooc/ec2-edx-configuration,hastexo/edx-configuration-old,appsembler/configuration,omarkhan/configuration,xiangjf/configuration,negritobomb/edxplat,B-MOOC/configuration,antoviaque/configuration,zhengjunwei/configuration,Unow/configuration,xiangjf/configuration,jorgeomarmh/configuration,hks-epod/configuration,cyanna/configuration,joshhu/configuration,kursitet/configuration,AlfiyaZi/configuration,Livit/Livit.Learn.EdX.configuration,antshin72/configuration,michaelsteiner19/open-edx-configuration,EDUlib/configuration,gashac03/ubuntu,nikolas/configuration,arbrandes/edx-configuration,marcore/configuration,alu042/configuration,pobrejuanito/configuration,open-craft/configuration,knehez/configuration,hmcmooc/ec2-edx-configuration,mitodl/configuration,arifsetiawan/configuration,jp-bpl/configuration,rue89-tech/configuration,proversity-org/configuration,EDUlib/configuration,apigee/edx-configuration,4eek/configuration,IndonesiaX/configuration,vasyarv/configuration,antoviaque/configuration,yrchen/configuration,Carlos2005/configuration,IONISx/edx-configuration,open-craft/configuration,louyihua/configuration,gsehub/configuration,edx/configuration,fghaas/edx-configuration,alu042/configuration,fghaas/edx-configuration,arbrandes/edx-configuration,gashac03/ubuntu,yrchen/configuration,armaan/edx-configuration,rue89-tech/configuration,nikolas/configuration,marcore/configuration,hastexo/edx-configuration,Carlos2005/configuration,joshhu/configuration,apigee/edx-configuration,alu042/configuration,kmoocdev/configuration,proversity-org/configuration,arifsetiawan/configuration,lgfa29/configuration,edx/configuration,nttks/configuration,antshin72/configuration,lgfa29/configuration,eduStack/configuration,pobrejuanito/configuration,pobrejuanito/configuration,CredoReference/configuration,openfun/configuration,mitodl/configuration,stvstnfrd/configuration,openfun/configuration,open-craft/configuration,hks-epod/configuration,chenshuchuan/configuration,openfun/configuration,nunpa/configuration,arifsetiawan/configuration,Pieros/configuration,proversity-org/configuration,nunpa/configuration,chenshuchuan/configuration,glengal/configuration,kencung/configuration,omarkhan/configuration,chudaol/configuration,LearnEra/LearnEra-Configuration,usernamenumber/configuration,4eek/configuration,kencung/configuration,negritobomb/edxplat,Unow/configuration,AlfiyaZi/configuration,hks-epod/configuration,mitodl/configuration,zhengjunwei/configuration,Stanford-Online/configuration,glengal/configuration,proversity-org/configuration,jp-bpl/configuration,kmoocdev/configuration,cecep-edu/configuration,appsembler/configuration,cecep-edu/configuration,antshin72/configuration,proversity-org/configuration,EDUlib/configuration,kursitet/configuration,antoviaque/configuration,omarkhan/configuration,gsehub/configuration,Livit/Livit.Learn.EdX.configuration,glengal/configuration,CredoReference/configuration,kmoocdev/configuration,leansoft/configuration,gsehub/configuration,Carlos2005/configuration,chudaol/configuration,rue89-tech/configuration,apigee/edx-configuration,antshin72/configuration,zhengjunwei/configuration,pedrorib/istx,joshhu/configuration,Pieros/configuration,hastexo/edx-configuration,bugcy013/edx_infra,nunpa/configuration,EDUlib/configuration,edx/configuration,beacloudgenius/configuration,chenshuchuan/configuration,stvstnfrd/configuration,fghaas/edx-configuration,beacloudgenius/configuration,appsembler/configuration,vasyarv/configuration,nttks/configuration,bugcy013/edx_infra,ovnicraft/evex-configuration,pedrorib/istx,Softmotions/configuration,gsehub/configuration,zhengjunwei/configuration,IndonesiaX/configuration,kursitet/configuration,B-MOOC/configuration,CredoReference/configuration,UXE/edx-configuration,xiangjf/configuration,eduStack/configuration,jp-bpl/configuration,omarkhan/configuration,UXE/edx-configuration,AlfiyaZi/configuration,B-MOOC/configuration,negritobomb/edxplat,cyanna/configuration,AlfiyaZi/configuration,hks-epod/configuration,stvstnfrd/configuration,yrchen/configuration,knehez/configuration,joshhu/configuration,marcore/configuration,alu042/configuration,kursitet/configuration,Softmotions/configuration,arifsetiawan/configuration,rue89-tech/configuration,openfun/configuration,michaelsteiner19/open-edx-configuration,kmoocdev/configuration,nunpa/configuration,gashac03/ubuntu,beacloudgenius/configuration,negritobomb/edxplat,Stanford-Online/configuration,Livit/Livit.Learn.EdX.configuration,kencung/configuration,cyanna/configuration,vasyarv/configuration,stvstnfrd/configuration,yrchen/configuration,louyihua/configuration,lgfa29/configuration,marcore/configuration,hastexo/edx-configuration,Softmotions/configuration,Stanford-Online/configuration,usernamenumber/configuration,louyihua/configuration,UXE/edx-configuration,IONISx/edx-configuration,jorgeomarmh/configuration,hastexo/edx-configuration-old,eduStack/configuration,michaelsteiner19/open-edx-configuration,EDUlib/configuration,nttks/configuration,sudheerchintala/LearnEra-Configuration,stvstnfrd/configuration,cyanna/configuration,Carlos2005/configuration,IONISx/edx-configuration,lgfa29/configuration,LearnEra/LearnEra-Configuration,UXE/edx-configuration,gsehub/configuration,Unow/configuration,hastexo/edx-configuration-old,leansoft/configuration,4eek/configuration,IndonesiaX/configuration,IONISx/edx-configuration,pedrorib/istx,fghaas/edx-configuration,jorgeomarmh/configuration,CredoReference/configuration,nttks/configuration,nikolas/configuration,knehez/configuration,leansoft/configuration,pobrejuanito/configuration,Softmotions/configuration,xiangjf/configuration,arbrandes/edx-configuration,jp-bpl/configuration,mitodl/configuration,ovnicraft/evex-configuration,leansoft/configuration,louyihua/configuration,Livit/Livit.Learn.EdX.configuration,Stanford-Online/configuration,arbrandes/edx-configuration,antoviaque/configuration,pedrorib/istx,armaan/edx-configuration,usernamenumber/configuration,sudheerchintala/LearnEra-Configuration,sudheerchintala/LearnEra-Configuration,arbrandes/edx-configuration,knehez/configuration,beacloudgenius/configuration,hmcmooc/ec2-edx-configuration,nikolas/configuration,Pieros/configuration,Pieros/configuration,jorgeomarmh/configuration