logind: do not fail creating a session when request is not from a unit

logind: do not fail creating a session when request is not from a unit

When running inside an LXC container the 'su' process will not be part of
any unit or slice.

manager_get_user_by_pid() which was used until v255 (included) does not fail
if it cannot find a unit/slice, but simply returns 'not found'. Do the same
in manager_get_session_by_pidref().

This was not detected as Semaphore CI does not reboot the testbed before
the logind test, so the session is started by the old logind from the base
distro, instead of the one being tested.

Follow-up for 8494f562c8963d8a936b0598e23eab277ff29374
Follow-up for 5099a50d4398e190387d204f5df81cc176bd33e2

Fixes https://github.com/systemd/systemd/issues/32929