fix: Filter only null values

Creates an AbstractEndpoint to provide a method
to filter only `null` values.

Classes/Endpoint/AbstractEndpoint.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Avency\Gitea\Endpoint;
+
+use Avency\Gitea\Client;
+
+/**
+ * Abstract endpoint
+ */
+abstract class AbstractEndpoint implements EndpointInterface
+{
+    /**
+     * @param array $array
+     * @return array
+     */
+    protected function removeNullValues(array $array): array
+    {
+        return array_filter(
+            $array,
+            function($value) {
+                return !is_null($value);
+            }
+        );
+    }
+}