Issue #3624439: Guard the click listener when the search tab is removed

Admin Toolbar Search 3.6.3 attaches a click listener to the search tab without checking whether it exists. A theme can remove that tab while retaining the autocomplete field, causing behavior attachment to throw. This guards only the optional tab listener. A disposable Chromium check using the shipped Drupal and jQuery assets confirms that the unpatched code throws when the tab is absent, the patched autocomplete still works after repeated behavior attachment, and a present tab retains focus and tray behavior. This complements the keyboard-shortcut fix in MR !217.

Closes #3624439