Fix uninitialized variable in transformTableLikeClause()

process_notnull_constraints should be set to false until we discover a NOT
NULL column.

Discovered while running Valgrind.

Discussion: https://postgr.es/m/CAApHDvoMyiZVi1KW5WVdqMRzWsWkD3F7n6QD+BbAO6WTeAWsUQ@mail.gmail.com

src/backend/parser/parse_utilcmd.c

@@ -1041,7 +1041,7 @@ transformTableLikeClause(CreateStmtContext *cxt, TableLikeClause *table_like_cla
 	AclResult	aclresult;
 	char	   *comment;
 	ParseCallbackState pcbstate;
-	bool		process_notnull_constraints;
+	bool		process_notnull_constraints = false;
 
 	setup_parser_errposition_callback(&pcbstate, cxt->pstate,
 									  table_like_clause->relation->location);