nss-systemd: properly handle empty membership lists

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 a1aa41e4e175c2712b97600d7e10e9d6c58e5543)
(cherry picked from commit d0f8a01a74e5a1bed4d687339abde774cb944579)