Rename NULL_MODEL macro

src/main.c

@@ -34,7 +34,7 @@ int main(void) {
   }
 
   Model obj = load_obj_file(arena, RESOURCE("head.obj"), RESOURCE("head.pnm"));
-  if (INVALID_MODEL(obj)) {
+  if (IS_INVALID_MODEL(obj)) {
     return TINY_EXIT_MODEL_LOAD_FAILED;
   }