Commit the auth associated changes I missed from the last commit.

Commit the auth associated changes I missed from the last commit.

Also set the default value of all the allocated strings to "" to avoid changing
the interface (becouse pdb_get...() would point to a null string, rather than a
null pointer and parts of samba rely on that).

Andrew Bartlett
(This used to be commit 5b4079f748e25f21162e21b439063249baf8dca6)