[Php81] Allow explicit mixed processed on trait on NullToStrictStringFuncCallArgRector

[samsonasik]

Explit mixed in trait:

trait ExplicitMixedInTrait
{
    public function setTitle(mixed $title)
    {
        $this->title = $title;
        return trim($this->title);
    }
}

should be allowed to be processed on NullToStrictStringFuncCallArgRector to be:

-return trim($this->title);
+return trim((string) $this->title);

[samsonasik]

All checks have passed 🎉 @TomasVotruba it is ready for review.

[samsonasik]

I am merging it ;)

[TomasVotruba]

Good job :)