[Php81] Handle crash insensitive function name on NullToStrictStringFuncCallArgRector

[samsonasik]

Given the following code:

final class CaseInsensitiveFunctionName
{
    public function run($var)
    {
        return strToLower($var);
    }
}

It currently make crash:

There was 1 error:

1) Rector\Tests\Php81\Rector\FuncCall\NullToStrictStringFuncCallArgRector\NullToStrictStringFuncCallArgRectorTest::test with data set #14 ('/Users/samsonasik/www/rector-...hp.inc')
Undefined array key "strToLower"

/Users/samsonasik/www/rector-src/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php:488
/Users/samsonasik/www/rector-src/rules/Php81/Rector/FuncCall/NullToStrictStringFuncCallArgRector.php:334

This PR fix it.

Fixes rectorphp/rector#7504

[samsonasik]

Fixed 🎉 /cc @noximo

[samsonasik]

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