[BUGFIX] Do not crash when moving empty folder

At least MinIO's list response does not contain a 'Content' property
when a folder has no child elements.

When moving a folder, check for its existence before iterating over it.

Resolves: #148

Classes/Driver/AmazonS3Driver.php

@@ -1487,7 +1487,7 @@ protected function resetIdentifierMap()
     protected function getSubObjects($identifier, $recursive = true, $filter = self::FILTER_ALL)
     {
         $result = $this->getListObjects($identifier);
-        if (!is_array($result['Contents'])) {
+        if (!is_array($result['Contents'] ?? null)) {
             return [];
         }
         return array_filter($result['Contents'], function (&$object) use ($identifier, $recursive, $filter) {