Support null attributes in code generated ExecVariableList
Support null attributes in code generated ExecVariableList

Enhacing code generated ExecVariableList with support of null attributes.
Also, there are some formatting changes based on Google C++ style.

jmcatamney/gpdb,rubikloud/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,Quikling/gpdb,adam8157/gpdb,kaknikhil/gpdb,xinzweb/gpdb,Chibin/gpdb,ahachete/gpdb,Chibin/gpdb,xuegang/gpdb,rubikloud/gpdb,rubikloud/gpdb,rubikloud/gpdb,edespino/gpdb,Quikling/gpdb,lintzc/gpdb,xuegang/gpdb,Chibin/gpdb,chrishajas/gpdb,Quikling/gpdb,CraigHarris/gpdb,ashwinstar/gpdb,edespino/gpdb,rvs/gpdb,Quikling/gpdb,lisakowen/gpdb,0x0FFF/gpdb,chrishajas/gpdb,xinzweb/gpdb,royc1/gpdb,Chibin/gpdb,janebeckman/gpdb,zaksoup/gpdb,0x0FFF/gpdb,CraigHarris/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,0x0FFF/gpdb,edespino/gpdb,xinzweb/gpdb,lintzc/gpdb,adam8157/gpdb,lintzc/gpdb,lintzc/gpdb,royc1/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,Chibin/gpdb,rvs/gpdb,xinzweb/gpdb,0x0FFF/gpdb,0x0FFF/gpdb,rubikloud/gpdb,jmcatamney/gpdb,lpetrov-pivotal/gpdb,cjcjameson/gpdb,xuegang/gpdb,50wu/gpdb,kaknikhil/gpdb,lpetrov-pivotal/gpdb,edespino/gpdb,zaksoup/gpdb,edespino/gpdb,lpetrov-pivotal/gpdb,jmcatamney/gpdb,lpetrov-pivotal/gpdb,xinzweb/gpdb,ashwinstar/gpdb,zaksoup/gpdb,xinzweb/gpdb,cjcjameson/gpdb,xuegang/gpdb,lintzc/gpdb,cjcjameson/gpdb,Chibin/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,Quikling/gpdb,chrishajas/gpdb,lpetrov-pivotal/gpdb,rvs/gpdb,janebeckman/gpdb,cjcjameson/gpdb,adam8157/gpdb,ashwinstar/gpdb,janebeckman/gpdb,yuanzhao/gpdb,chrishajas/gpdb,Chibin/gpdb,lintzc/gpdb,lpetrov-pivotal/gpdb,zaksoup/gpdb,Quikling/gpdb,janebeckman/gpdb,ahachete/gpdb,zaksoup/gpdb,rubikloud/gpdb,royc1/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,janebeckman/gpdb,edespino/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,adam8157/gpdb,CraigHarris/gpdb,royc1/gpdb,50wu/gpdb,lintzc/gpdb,CraigHarris/gpdb,ahachete/gpdb,yuanzhao/gpdb,rvs/gpdb,CraigHarris/gpdb,chrishajas/gpdb,ahachete/gpdb,royc1/gpdb,xuegang/gpdb,janebeckman/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ahachete/gpdb,50wu/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ahachete/gpdb,cjcjameson/gpdb,zaksoup/gpdb,edespino/gpdb,yuanzhao/gpdb,xuegang/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,lisakowen/gpdb,Quikling/gpdb,rvs/gpdb,lisakowen/gpdb,greenplum-db/gpdb,Chibin/gpdb,adam8157/gpdb,Chibin/gpdb,jmcatamney/gpdb,janebeckman/gpdb,edespino/gpdb,50wu/gpdb,adam8157/gpdb,yuanzhao/gpdb,lpetrov-pivotal/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,ahachete/gpdb,ashwinstar/gpdb,royc1/gpdb,xuegang/gpdb,chrishajas/gpdb,royc1/gpdb,lintzc/gpdb,kaknikhil/gpdb,kaknikhil/gpdb,chrishajas/gpdb,janebeckman/gpdb,royc1/gpdb,Quikling/gpdb,jmcatamney/gpdb,50wu/gpdb,rvs/gpdb,cjcjameson/gpdb,chrishajas/gpdb,yuanzhao/gpdb,0x0FFF/gpdb,adam8157/gpdb,0x0FFF/gpdb,edespino/gpdb,lpetrov-pivotal/gpdb,janebeckman/gpdb,rvs/gpdb,lisakowen/gpdb,ahachete/gpdb,Quikling/gpdb,jmcatamney/gpdb,adam8157/gpdb,yuanzhao/gpdb,Chibin/gpdb,CraigHarris/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,50wu/gpdb,janebeckman/gpdb,CraigHarris/gpdb,rvs/gpdb,xinzweb/gpdb,zaksoup/gpdb,CraigHarris/gpdb,rubikloud/gpdb,lintzc/gpdb,kaknikhil/gpdb,edespino/gpdb,rvs/gpdb,greenplum-db/gpdb,xuegang/gpdb,rvs/gpdb,greenplum-db/gpdb,rubikloud/gpdb,0x0FFF/gpdb,Quikling/gpdb,ashwinstar/gpdb,50wu/gpdb,zaksoup/gpdb,xuegang/gpdb,50wu/gpdb