nixos/users-groups: Fix eval on missing uid/gid.

This hopefully fixes a regression introduced by 08b214a.

In bf129a2, it was already fixed for normal uid/gid values and it got
reintroduced by sub-uid/gid-handling again, so I've refactored it a bit
into a filterNull function which takes care of also the filtering
introduced by bf129a2.

I have not tested this extensively, but master is already broken for
systems with `mutableUsers = true` and no uid values set.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>