Fix TypeError: Cannot read properties of null (reading 'finished')

Add null check for response before accessing finished property in PUT endpoint.
When getResponse returns null (response not found), return 404 instead of
attempting to access properties on null object.

Fixes FORMBRICKS-FN

apps/web/app/api/v1/client/[environmentId]/responses/[responseId]/route.ts

@@ -71,6 +71,12 @@ export const PUT = withV1ApiWrapper({
       };
     }
 
+    if (!response) {
+      return {
+        response: responses.notFoundResponse("Response", responseId, true),
+      };
+    }
+
     if (response.finished) {
       return {
         response: responses.badRequestResponse("Response is already finished", undefined, true),