[BUGFIX] Fixed deprecation notice in AbstractSecurityViewHelper

[evorg]

Fixed deprecation notice in AbstractSecurityViewHelper for implicitly marking parameter $frontendUsers as nullable:

  [ TYPO3\CMS\Core\Error\Exception ]                                           
  PHP Runtime Deprecation Notice: FluidTYPO3\Vhs\ViewHelpers\Security\Abstrac  
  tSecurityViewHelper::assertFrontendUsersLoggedIn(): Implicitly marking para  
  meter $frontendUsers as nullable is deprecated, the explicit nullable type   
  must be used instead in /var/www/app/vendor/fluidtypo3/vhs/Classes/ViewHelp  
  ers/Security/AbstractSecurityViewHelper.php line 233

$frontendUsers is now explicitly nullable.