[TypeDeclaration] Skip union mixed on TypedPropertyFromAssignsRector

[samsonasik]

Given the following code:

final class SkipUnionMixed
{
    private $x;

    public function __construct($x)
    {
        if (rand(0,1)) {
            $this->x = $x;
        } else {
            $this->x = null;
        }
    }
}

It currently produce:

+    /**
+     * @var mixed|null
+     */
     private $x;

which should be skipped instead.

Fixes rectorphp/rector#7640

[samsonasik]

Fixed 🎉

[samsonasik]

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

[samsonasik]

I Moved logic to new service: NewPhpDocFromPHPStanTypeGuard so it can be re-use and added more check if needed 0652761