Allow background workers to connect to no particular database.
Allow background workers to connect to no particular database.

The documentation claims that this is supported, but it didn't
actually work.  Fix that.

Reported by Pavel Stehule; patch by me.

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