Bug 1762688 follow-up - Fix eslint error. r=#thunderbird-reviewers,john.bieling

Differential Revision: https://phabricator.services.mozilla.com/D157289

--HG--
extra : amend_source : f2755de38501d98c903fe5693ef565f583490f0c

mailnews/imap/src/ImapIncomingServer.jsm

@@ -450,6 +450,7 @@ class ImapIncomingServer extends MsgIncomingServer {
     if (canGetNext) {
       return this._getNextClient();
     }
+    return null;
   }
 
   /**