Another PHP 8.1 fix

Fixes this deprecation warning:
```
PHP Deprecated:  strtoupper(): Passing null to parameter #1 ($string) of type string is deprecated in /srv/matomo/vendor/matomo/matomo-php-tracker/MatomoTracker.php on line 1623
```

MatomoTracker.php

@@ -1620,7 +1620,7 @@ protected function sendRequest($url, $method = 'GET', $data = null, $force = fal
 
         $forcePostUrlEncoded = false;
         if (!$this->doBulkRequests) {
-            if (strtoupper($this->requestMethod) === 'POST') {
+            if (!empty($this->requestMethod) && strtoupper($this->requestMethod) === 'POST') {
                 // POST ALL parameters and have no GET parameters
                 $urlParts = explode('?', $url);