Deprecation Messages in PHP 8.1

[JEreth]

When using Flux with PHP 8.1 we receive these deprecation messages when we are in dev mode. The messages are caused by the deprecation of passing null values to non-nullable internal function parameters (see https://php.watch/versions/8.1/internal-func-non-nullable-null-deprecation).

PHP Runtime Deprecation Notice: class_exists(): Passing null to parameter #1 ($class) of type string is deprecated in /public/typo3conf/ext/flux/Classes/Controller/AbstractFluxController.php line 409

 PHP Runtime Deprecation Notice: urldecode(): Passing null to parameter #1 ($string) of type string is deprecated in /public/typo3conf/ext/flux/Classes/Integration/HookSubscribers/ContentIcon.php line 177

See PR for a potential fix.