Fix a lot of pointer comparisons to 0 (or nothing) to explicitly use NULL

src/script.c

@@ -349,7 +349,7 @@ void ExecuteScriptForEntity(Entity* entity, void (*postScriptCallback)(Entity*,
         } else {
             HandlePostScriptActions(entity, *piVar1);
         }
-        if (!entity->next) {
+        if (entity->next == NULL) {
             DeleteThisEntity();
         }
     }