Fix JS error on compare page (#23551) (#23566)

Backport #23551 by @silverwind

Fix regression from https://github.com/go-gitea/gitea/pull/23271. This
code apparently runs on the compare page where there is no review button
so check for existance of both button and content.

<img width="1226" alt="Screenshot 2023-03-17 at 21 42 47"
src="https://user-images.githubusercontent.com/115237/226040001-ec097edd-d926-4c80-9962-526f9b7eff19.png">

Fixes #23562

Co-authored-by: silverwind <me@silverwind.io>

web_src/js/features/repo-issue.js

@@ -515,6 +515,7 @@ export function initRepoPullRequestReview() {
   const $panel = $reviewBtn.parent().find('.review-box-panel');
   const $closeBtn = $panel.find('.close');
 
+  if ($reviewBtn.length && $panel.length) {
     const tippy = createTippy($reviewBtn[0], {
       content: $panel[0],
       placement: 'bottom',
@@ -529,6 +530,7 @@ export function initRepoPullRequestReview() {
       e.preventDefault();
       tippy.hide();
     });
+  }
 
   $(document).on('click', 'a.add-code-comment', async function (e) {
     if ($(e.target).hasClass('btn-add-single')) return; // https://github.com/go-gitea/gitea/issues/4745