disable test for `null.nonExistent()` on lua and as3

tests/unit/src/unit/issues/Issue4988.hx

@@ -9,8 +9,8 @@ class Issue4988 extends Test {
 			try {
 				var d:{i:Null<Int>} = null;
 				value = (d.i > 0);
-				(null:Dynamic).nonExistent();
 				#if !(lua || as3)
+				(null:Dynamic).nonExistent();
 				null.nonExistent();
 				#end
 				t(false);