Passing NULL to \Drupal\Component\Utility\Unicode::truncate() causes deprecations on PHP 8.1

I disagree very much with #13. If we fix this in Unicode::truncate() we can't add a proper scalar typehint to it. Please revert to #2.

RE-upping #2 and fixing the issue summary. If we change Unicode::truncate() then we lose the deprecation if you pass it NULL. If you pass NULL to Unicode::truncate then almost certainly your code is not behaving as you expect it to. Ie. maybe you don't have a label when you expect one.

I wrote the original fix as part #3220021: [meta] Ensure compatibility of Drupal 9 with PHP 8.1 (as it evolves)

We can combine the two related fixes.

This is a migration. Let's take the opportunity to do less work.

In fact the menu link example is a case in point. If you create a menu link without a description via the UI then in the DB the description column is NULL... so if we have no description we should not be calling truncate on it and turning it into an empty string...

I created a menu link via the UI and then did the following select on the DB.

mysql> select title, link__uri, description from menu_link_content_data;
+----------------+--------------------+-------------+
| title          | link__uri          | description |
+----------------+--------------------+-------------+
| No description | http://example.com | NULL        |
+----------------+--------------------+-------------+
1 row in set (0.00 sec)

Here's the fix for #18 - let's see if any tests break.

@andypost I think strict type hints on things like Unicode::truncate(), Html::escape() etc... are part of the scope of adding scalar typehints to core. There's no need for a specific followup here as that issue is massive and has several existing issues (I think).

We need to add test coverage for descriptions staying NULL during a migration.

@andypost I think we should leave that alone until we have a plan for scalar typehints. Like we might issue our deprecation so adding an assert is not necessary. Plus if you test on PHP 8.1 you get that for free right now :)

Back to needs work for the needed tests.

@andypost we need an assertion somewhere that description is NULL (and not an empty string). FWIW the change in #28 doesn't fail on HEAD.

I've step debugged through the code and realised that the change here does not need a test. I incorrectly assumed that Unicode:truncate(NULL) would return an empty string. It does not it returns a NULL (and on PHP 8.1 it'll issue a deprecation as this occurs because we pass NULL into mb_strlen()). Therefore, even on HEAD description is set to NULL if options/attributes/title is not set. So the patch in #19 is good to go here. Re-uploading. The test change in #28 passes on HEAD so it's not really telling us anything new.

Committed caf0971 and pushed to 9.3.x. Thanks!