Contao\CoreBundle\Security\Authentication\ContaoToken::getCredentials PHP Method

    public function getCredentials()
    {
        return '';
    }

 /**
  * Tests a back end user.
  *
  * @runInSeparateProcess
  * @preserveGlobalState disabled
  */
 public function testBackendUser()
 {
     $token = new ContaoToken(BackendUser::getInstance());
     $this->assertTrue($token->isAuthenticated());
     $this->assertEquals('', $token->getCredentials());
     $this->assertEquals([new Role('ROLE_USER'), new Role('ROLE_ADMIN')], $token->getRoles());
 }