nss-systemd: properly handle empty membership lists

When we are queried for membership lists on a system that has exactly
zero, then we'll return ESRCH immediately instead of at EOF. Which is
OK, but we need to handle this in various places, and not get confused
by it.

(cherry picked from commit a1aa41e4)
(cherry picked from commit d0f8a01a)
(cherry picked from commit 83c39430)