[9.x] Made Http::sequence()->push() consistent with Http::response() (#42659)

* Made Http::sequence()->push() consistent with Http::response() by allowing null as the body. This will also fix an error in PHPStan.

* changed type

Author: SuperDJ

src/Illuminate/Http/Client/Factory.php

@@ -119,7 +119,7 @@ public function __construct(Dispatcher $dispatcher = null)
     /**
      * Create a new response instance for use during stubbing.
      *
-     * @param  array|string  $body
+     * @param  array|string|null  $body
      * @param  int  $status
      * @param  array  $headers
      * @return \GuzzleHttp\Promise\PromiseInterface

src/Illuminate/Http/Client/ResponseSequence.php

@@ -44,12 +44,12 @@ public function __construct(array $responses)
     /**
      * Push a response to the sequence.
      *
-     * @param  string|array  $body
+     * @param  string|array|null  $body
      * @param  int  $status
      * @param  array  $headers
      * @return $this
      */
-    public function push($body = '', int $status = 200, array $headers = [])
+    public function push($body = null, int $status = 200, array $headers = [])
     {
         $body = is_array($body) ? json_encode($body) : $body;