[4.3] Tags: Don't put NULL into trim()

[Hackwar]

Pull Request for Issue #41466.

Summary of Changes

When on PHP 8.1 and editing an article, adding an unknown tag triggers an AJAX call which throws a notice.

Testing Instructions

1. You have to be on PHP 8.1 or higher
2. Edit an article and click into the tags field
3. Open the browser console to see the ajax request
4. Type a random tag-name into the tag field
5. See the request in the console and check the request answer

Actual result BEFORE applying this Pull Request

A notice is thrown and the JS actually throws an error, too.

Expected result AFTER applying this Pull Request

No notice, everything is hunky dorry. 😉

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[heelc29]

I have tested this item ✅ successfully on 149d153

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41509.}

[chmst]

I have tested this item ✅ successfully on 149d153

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41509.}

[alikon]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41509.}

[wilsonge]

Thanks!