fix: Allow usage of null parameter to pass optional url argument

Previously, if arguments pass as `$api->projects($idOrName)` were failing if `$idOrName` was null, expecting a scalar type, now ignore arguments if it's value is null.

This allow to use `$api->projects($idOrName)`:
* to request `/projects` when `$idOrName` is null
* to request `/projects/{idOrName}` otherwise

src/GitlabApiClient.php

@@ -95,6 +95,10 @@ public function __call($method, array $args)
         }
 
         foreach ($args as $param) {
+            if (null === $param) {
+                continue;
+            }
+
             if (!is_scalar($param)) {
                 throw new \InvalidArgumentException(sprintf('Method "%s" : Invalid argument type. scalar expected.', $method));
             }