452 failing tests due to wrong use of cache key

[imath]

I believe the reason is in ​WP's 53818 changeset. The WP Cache API is less permissive in WordPress 6.1 and throws "Unexpected incorrect usage notice" when the cache key is not an integer nor a not empty string.

After a analysing the 2 first failing tests it looks like it corresponds to:

• an activity comment created without the parent activity inside the test (not a regular usage).
• a group is created and the membership id is null for the creator at the time the 'groups_member_before_save' hook is fired (regular usage)

I'm adding a first patch about it to illustrate the 2 fixes.

450 failing tests left to fix.. which is a lot!

[imath]

Well it was more impressive than critical 😅. ​.2.patch is fixing the issue.

[imath]

In 13309:

Adapt to WP Cache API latest change about cache key requirements

In WordPress 6.1, a cache key needs to be an integer or a "not empty" string. See ​WP53818 changeset.

After analysing the failing PHPUnit failing tests, it seems we were not careful enough about cache key checks in a few cases:

• When generating an activity comment without a corresponding parent activity in our tests.
• When trying to cache a group membership ID for the group creator before this membership has been created.
• When trying to check if a member had a member type or a group had a group type using an empty string although into 2 of our unit tests.

Fixes #8727