Use ResponseSequence::isEmpty() (#45757)

The class offers a `isEmpty()` method to check if the client has no responses anymore. Use that method to keep the implementation details in one place.

Author: Konafets

Client/ResponseSequence.php

@@ -143,11 +143,11 @@ public function isEmpty()
      */
     public function __invoke()
     {
-        if ($this->failWhenEmpty && count($this->responses) === 0) {
+        if ($this->failWhenEmpty && $this->isEmpty()) {
             throw new OutOfBoundsException('A request was made, but the response sequence is empty.');
         }
 
-        if (! $this->failWhenEmpty && count($this->responses) === 0) {
+        if (! $this->failWhenEmpty && $this->isEmpty()) {
             return value($this->emptyResponse ?? Factory::response());
         }