Remove default_branch from project API creation since an empty repository has no branches to start.
Remove default_branch from project API creation since an empty repository
has no branches to start.

Closes #3937

shinexiao/gitlabhq,daiyu/gitlab-zh,jrjang/gitlabhq,mrb/gitlabhq,cui-liqiang/gitlab-ce,martijnvermaat/gitlabhq,yatish27/gitlabhq,Soullivaneuh/gitlabhq,jrjang/gitlabhq,daiyu/gitlab-zh,icedwater/gitlabhq,SVArago/gitlabhq,koreamic/gitlabhq,jrjang/gitlabhq,ttasanen/gitlabhq,axilleas/gitlabhq,mrb/gitlabhq,mrb/gitlabhq,htve/GitlabForChinese,daiyu/gitlab-zh,SVArago/gitlabhq,duduribeiro/gitlabhq,LUMC/gitlabhq,larryli/gitlabhq,iiet/iiet-git,Datacom/gitlabhq,axilleas/gitlabhq,darkrasid/gitlabhq,screenpages/gitlabhq,mr-dxdy/gitlabhq,louahola/gitlabhq,martijnvermaat/gitlabhq,louahola/gitlabhq,shinexiao/gitlabhq,screenpages/gitlabhq,mmkassem/gitlabhq,larryli/gitlabhq,dwrensha/gitlabhq,jirutka/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,jrjang/gitlab-ce,mmkassem/gitlabhq,ttasanen/gitlabhq,stoplightio/gitlabhq,htve/GitlabForChinese,duduribeiro/gitlabhq,mr-dxdy/gitlabhq,LUMC/gitlabhq,dreampet/gitlab,louahola/gitlabhq,larryli/gitlabhq,mmkassem/gitlabhq,duduribeiro/gitlabhq,ttasanen/gitlabhq,iiet/iiet-git,dreampet/gitlab,martijnvermaat/gitlabhq,Datacom/gitlabhq,mmkassem/gitlabhq,koreamic/gitlabhq,allysonbarros/gitlabhq,yatish27/gitlabhq,dplarson/gitlabhq,jrjang/gitlabhq,Soullivaneuh/gitlabhq,allysonbarros/gitlabhq,louahola/gitlabhq,dplarson/gitlabhq,LUMC/gitlabhq,jirutka/gitlabhq,yatish27/gitlabhq,shinexiao/gitlabhq,stoplightio/gitlabhq,Datacom/gitlabhq,allysonbarros/gitlabhq,mr-dxdy/gitlabhq,cui-liqiang/gitlab-ce,SVArago/gitlabhq,t-zuehlsdorff/gitlabhq,icedwater/gitlabhq,ttasanen/gitlabhq,daiyu/gitlab-zh,larryli/gitlabhq,dwrensha/gitlabhq,jrjang/gitlab-ce,Soullivaneuh/gitlabhq,openwide-java/gitlabhq,dplarson/gitlabhq,LUMC/gitlabhq,stoplightio/gitlabhq,yatish27/gitlabhq,martijnvermaat/gitlabhq,koreamic/gitlabhq,jrjang/gitlab-ce,iiet/iiet-git,jirutka/gitlabhq,mr-dxdy/gitlabhq,t-zuehlsdorff/gitlabhq,screenpages/gitlabhq,duduribeiro/gitlabhq,dwrensha/gitlabhq,icedwater/gitlabhq,axilleas/gitlabhq,jrjang/gitlab-ce,jirutka/gitlabhq,dreampet/gitlab,cui-liqiang/gitlab-ce,stoplightio/gitlabhq,mrb/gitlabhq,cui-liqiang/gitlab-ce,allysonbarros/gitlabhq,koreamic/gitlabhq,openwide-java/gitlabhq,iiet/iiet-git,htve/GitlabForChinese,icedwater/gitlabhq,t-zuehlsdorff/gitlabhq,openwide-java/gitlabhq,Datacom/gitlabhq,htve/GitlabForChinese,dwrensha/gitlabhq,SVArago/gitlabhq,dplarson/gitlabhq,darkrasid/gitlabhq,darkrasid/gitlabhq,shinexiao/gitlabhq,Soullivaneuh/gitlabhq,openwide-java/gitlabhq,screenpages/gitlabhq,darkrasid/gitlabhq,axilleas/gitlabhq