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: #293

Author: cweiske

src/Runtime/Http/Requests.php

@@ -134,7 +134,7 @@ private static function init(RequestOptions $options)
             $opt = $options->Method === HttpMethod::Get ? CURLOPT_FILE : CURLOPT_INFILE;
             curl_setopt($ch, $opt, $options->StreamHandle);
         }
-        $options->ensureHeader("Content-Length",strlen($options->Data));
+        $options->ensureHeader("Content-Length",strlen((string) $options->Data));
         //custom HTTP headers
         if($options->Headers)
             curl_setopt($ch, CURLOPT_HTTPHEADER, $options->getRawHeaders());