Fix "str_starts_with(): Passing null to parameter #1 ($haystack) of t…

…ype string is deprecated"

Signed-off-by: Joas Schilling <coding@schilljs.com>

tests/integration/features/bootstrap/FeatureContext.php

@@ -3996,7 +3996,7 @@ public function sendRequestFullUrl($verb, $fullUrl, $body = null, array $headers
  $options = array_merge($options, ['cookies' => $this->getUserCookieJar($this->currentUser)]);
  if ($this->currentUser === 'admin') {
  $options['auth'] = ['admin', 'admin'];
- } elseif (str_starts_with($this->currentUser, 'federation')) {
+ } elseif ($this->currentUser !== null && str_starts_with($this->currentUser, 'federation')) {
  $auth = explode('#', $this->currentUser);
  array_shift($auth);
  $options['auth'] = $auth;