Fix creative, but unportable, spelling of "ptr != NULL".

Or at least I suppose that's what was really meant here.  But even
aside from the not-per-project-style use of "0" to mean "NULL",
I doubt it's safe to assume that all valid pointers are > NULL.
Per buildfarm member pademelon.

src/backend/commands/tablecmds.c

@@ -13167,7 +13167,7 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd)
 	tupleDesc = RelationGetDescr(attachRel);
 	attachRel_constr = tupleDesc->constr;
 	existConstraint = NIL;
-	if (attachRel_constr > 0)
+	if (attachRel_constr != NULL)
 	{
 		int			num_check = attachRel_constr->num_check;
 		int			i;