Update validateAttribute for PHP 8.1

[samuelexyz]

In function validateAttribute call_user_func does not control if $value is null

Added control to fix

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues

[yii-bot]

Thank you for putting effort in the improvement of the Yii framework.
We have reviewed your pull request.

Unfortunately a use case is missing. It is required to get a better understanding of the pull request and helps us to determine the necessity and applicability of the suggested change to the framework.

Could you supply us with a use case please? Please be as detailed as possible and show some code!

Thanks!

This is an automated comment, triggered by adding the label pr:missing usecase.

[samuelexyz]

In PHP 8.1 we need to check if argument of a function is not null, when try to generate code with gii throw an error

PHP Deprecated Warning – yii\base\ErrorException
trim(): Passing null to parameter #1 ($string) of type string is deprecated

[samdark]

Please add a line for CHANGELOG. Thanks.

[samuelexyz]

@samdark Added to Changelog

Thanks

[samdark]

Thanks.

[samdark]

Hmm... seems it broke existing tests: d056c7f

[samdark]

Re-checked and reverted. The error depends on what's in the $this->filter so it's a case specific to Gii and should be fixed there instead.