postgres_fdw: Fix incorrect NULL handling in join pushdown.

something.* IS NOT NULL means that every attribute of the row is not
NULL, not that the row itself is non-NULL (e.g. because it's coming
from below an outer join.  Use (somevar.*)::pg_catalog.text IS NOT
NULL instead.

Ashutosh Bapat, per a report by Rushabh Lathia.  Reviewed by
Amit Langote and Etsuro Fujita.  Schema-qualification added by me.

Author: robertmhaas

contrib/postgres_fdw/deparse.c

@@ -1599,9 +1599,9 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
 
 		if (qualify_col)
 		{
-			appendStringInfoString(buf, "CASE WHEN ");
+			appendStringInfoString(buf, "CASE WHEN (");
 			ADD_REL_QUALIFIER(buf, varno);
-			appendStringInfo(buf, "* IS NOT NULL THEN %u END", fetchval);
+			appendStringInfo(buf, "*)::pg_catalog.text IS NOT NULL THEN %u END", fetchval);
 		}
 		else
 			appendStringInfo(buf, "%u", fetchval);
@@ -1643,9 +1643,9 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
 		 */
 		if (qualify_col)
 		{
-			appendStringInfoString(buf, "CASE WHEN ");
+			appendStringInfoString(buf, "CASE WHEN (");
 			ADD_REL_QUALIFIER(buf, varno);
-			appendStringInfo(buf, "* IS NOT NULL THEN ");
+			appendStringInfo(buf, "*)::pg_catalog.text IS NOT NULL THEN ");
 		}
 
 		appendStringInfoString(buf, "ROW(");