Skip null groups in group manager (#26871) (#26956)

* Skip null groups in group manager (#26871)

* Skip null groups in group manager

* Also skip null groups in group manager's search function

* Add more group null checks in sharing code

* Add unit tests for null group safety in group manager

* Add unit tests for sharing code null group checks

* Added tests for null groups handling in sharing code

* Ignore moveShare optional repair in mount provider

In some cases, data is inconsistent in the oc_share table due to legacy
data. The mount provider might attempt to make it consistent but if the
target group does not exist any more it cannot work. In such case we
simply ignore the exception as it is not critical. Keeping the
exception would break user accounts as they would be unable to use
their filesystem.

* Adjust null group handing + tests

* Fix new group manager tests

Signed-off-by: Morris Jobke <hey@morrisjobke.de>