Avoid JS error on issue/pr list when logged out (#29854)

When logged out, the checkboxes are not there on the issue/pr lists,
which would cause an error here.

Fixes: https://github.com/go-gitea/gitea/issues/29862

---------

Co-authored-by: delvh <dev.lh@web.de>

web_src/js/features/repo-issue-list.js

@@ -9,6 +9,7 @@ import {DELETE, POST} from '../modules/fetch.js';
 
 function initRepoIssueListCheckboxes() {
   const issueSelectAll = document.querySelector('.issue-checkbox-all');
+  if (!issueSelectAll) return; // logged out state
   const issueCheckboxes = document.querySelectorAll('.issue-checkbox');
 
   const syncIssueSelectionState = () => {