[BUGFIX] Fix TypeError in HeadlessUserInt::unwrap() when preg_replace_callback returns null

[Rhovan2223]

Problem

HeadlessUserInt::unwrap() declares string as its return type, but preg_replace_callback() can return null on internal failure (as documented in the PHP manual).

This causes a sporadic TypeError in production environments:
TypeError: FriendsOfTYPO3\Headless\Utility\HeadlessUserInt::unwrap(): Return value must be of type string, null returned
The issue is intermittent and hard to reproduce, but occurs under real-world load.

Fix

Add the null coalescing operator (?? $content) to all four preg_replace_callback() calls in unwrap(), so the original content is preserved instead of returning null.

This is the minimal, non-breaking fix — no behavioral change when preg_replace_callback() succeeds.

Changes

• Classes/Utility/HeadlessUserInt.php: Add ?? $content to all 4 preg_replace_callback() calls

[twoldanski]

@Rhovan2223 Hi thank you for report and patch! FYI I incorporated your patch into mine #869, as fallback. (good catch!) We were not hitting these limits so bug was flying under the radar, but you are correct in some cases you can hit it, and then crash...

In my patch I added some optimizations which are trying to limit hitting php errors (and return nulls), if you can tests on your project if it helps you, it would be much appreciated

[Rhovan2223]

Thanks, will test once merged!