php 8.1: Argument #1 ($rpc_responses) must be of type array, null given

src/Controller/HttpController.php

@@ -162,15 +162,14 @@ class HttpController extends ControllerBase {
     try {
       $rpc_responses = $this->getRpcResponses($rpc_requests);
 
-      // Aggregate the response headers so we can add them to the HTTP response.
-      $header_bag = $this->aggregateResponseHeaders($rpc_responses);
-
       // If no RPC response(s) were generated (happens if all of the request(s)
       // were notifications), then return a 204 HTTP response.
       if (empty($rpc_responses)) {
-        $response = new CacheableJsonResponse(NULL, Response::HTTP_NO_CONTENT);
-        $response->headers->add($header_bag->all());
-        return $response;
+        return new CacheableJsonResponse(NULL, Response::HTTP_NO_CONTENT);
+      }
+      else {
+        // Aggregate the response headers to add them to the HTTP response.
+        $header_bag = $this->aggregateResponseHeaders($rpc_responses);
       }
 
       // Map the RPC response(s) to an HTTP response.