Fix error on getUid(null) with 0 results (Webklex#499)

Author: Wiebe Haanstra

src/Connection/Protocols/ImapProtocol.php

@@ -939,7 +939,7 @@ public function getUid(?int $id = null): Response {
         $uids = $this->uid_cache;
 
         if ($id == null) {
-            return Response::empty($this->debug)->setResult($uids);
+            return Response::empty($this->debug)->setResult($uids)->setCanBeEmpty(true);
         }
 
         foreach ($uids as $k => $v) {