Bug 1752082 - follow-up: Fix unsubscribing newsgroup when NNTP server is the first account in folderPane. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D138754

--HG--
extra : amend_source : 7274c2ef1f1da31b50cf0ba9e4693ea000b7e908

mail/base/content/folderPane.js

@@ -2829,7 +2829,7 @@ var gFolderTreeView = {
 
     if (aParentItem) {
       let parentIndex = this.getIndexOfFolder(aParentItem);
-      if (!parentIndex) {
+      if (parentIndex == null) {
         return;
       }
       if (!this._rowMap[parentIndex].open) {