My fix for #24272, #23613, File not found when single file in a folder (#24800)

* Fixed group share searching for members of
multiple group. Issue #11808.

* Fixed group share searching, continued.

Avoid searching for empty group list in getItemSharedWithUser().
Broke tests in previous commit, #12030.

* Simler check for group count.

* Fix for https://github.com/owncloud/core/issues/24783 , described there

* Now it's #24272, 24783 was a duplicate. Previous change was also not very good. Now we don't create ZIP with a single file inside.