[10.x] Tweak return type for missing config

[sfreytag]

My subclass of PasswordBrokerManager fails with Larastan with the error:

 ------ ---------------------------------------------------------------------- 
  Line   Services/MyAppPasswordBrokerManager.php                              
 ------ ---------------------------------------------------------------------- 
  22     Call to function is_null() with array will always evaluate to false.  
 ------ ---------------------------------------------------------------------- 

Line 22 is the same as the framework version:

        if (is_null($config)) {
            throw new InvalidArgumentException("Password resetter [{$name}] is not defined.");
        }

I think it's correct that $config can be null and this docblock tweak fixes that for Larastan.

[driesvints]

Thanks!