Use identity for None comparison
Use identity for None comparison

It's recommended to compare identity against None with 'is' or 'is not'
rather than by equality with the '==' or '!=' operators, even if custom
comparison operators aren't implemented. 'is' conveys meaning, and is a
hair faster (which in this case is irrelevant).

Reviewed-by: Abhijit Subramanya

lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb