Return NULL for wildcard values in getnetgrent from nscd (BZ #16759)

getnetgrent is supposed to return NULL for values that are wildcards
in the (host, user, domain) triplet.  This works correctly with nscd
disabled, but with it enabled, it returns a blank ("") instead of a
NULL.  This is easily seen with the output of `getent netgroup foonet`
for a netgroup foonet defined as follows in /etc/netgroup:

    foonet (,foo,)

The output with nscd disabled is:

    foonet ( ,foo,)

while with nscd enabled, it is:

    foonet (,foo,)

The extra space with nscd disabled is due to the fact that `getent
netgroup` adds it if the return value from getnetgrent is NULL for
either host or user.

(cherry picked from commit dd3022d7)