Fix Deprecated: strpos(): Passing null to parameter in PHP 8.1 fixes d…

…palou#5.

filter.php

@@ -178,7 +178,7 @@ protected function is_ws_access() {
         }
 
         // Check rare cases, like webservice/pluginfile.php.
-        if (strpos($ME, 'webservice/') !== false) {
+        if (!is_null($ME) && strpos($ME, 'webservice/') !== false) {
             $token = optional_param('token', '', PARAM_ALPHANUM);
             if ($token) {
                 return true;