Change one more check to not accept NULL result from MultiExecProcNode().
Change one more check to not accept NULL result from MultiExecProcNode().

I missed this in commit b219c1bfbc.

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