Fix uninitilized variable (#4293)

Valgrind found that, we were doing an if check on uninitialized variable
and it seems that this is on context.appendparents.

ac22929a26/src/backend/utils/adt/ruleutils.c (L1054)

src/backend/distributed/deparser/ruleutils_13.c

@@ -7763,6 +7763,7 @@ pg_get_triggerdef_worker(Oid trigid, bool pretty)
 		context.wrapColumn = WRAP_COLUMN_DEFAULT;
 		context.indentLevel = PRETTYINDENT_STD;
 		context.special_exprkind = EXPR_KIND_NONE;
+		context.appendparents = NULL;
 
 		get_rule_expr(qual, &context, false);