Add empty reason phrase when returning a response.

Instead of omitting the parameter, provide an empty string to restore default behaviour of looking it up. This works around a diactoros bug.

src/Middleware/ErrorDocumentSetup.php

@@ -57,7 +57,7 @@ public function process(Request $request, DelegateInterface $delegate) : Respons
         }
 
         $response = $delegate->handle($request);
-        return $response->withStatus(404);
+        return $response->withStatus(404, '');
     }
 
     /**

src/Middleware/ErrorResponseGenerator.php

@@ -38,10 +38,10 @@ public function __invoke(Throwable $error, Request $request, Response $response)
         try {
             if ($error instanceof DocumentNotFoundException) {
                 $response = $this->notFoundPipeline->process($request, $this);
-                return $response->withStatus(404);
+                return $response->withStatus(404, '');
             }
             $response = $this->errorPipeline->process($request, $this);
-            return $response->withStatus(500);
+            return $response->withStatus(500, '');
         } catch (\Throwable $e) {
             return $this->generateFallbackResponse();
         }