Fix deprecation: strlen with null
Fix deprecation: strlen with null

PHP 8.1 logs a deprecation message:
> PHP Deprecated:  strlen(): Passing null to parameter #1 ($string)
> of type string is deprecated
> in vendor/vgrem/php-spo/src/Runtime/Http/Requests.php on line 137

We fix that by casting the variable to string.

Related: https://github.com/vgrem/phpSPO/issues/293

vgrem/phpSPO