pwdb_smb_to_sam was not returning NULL for nt name so that

pwdb_smb_to_sam was not returning NULL for nt name so that
pwdb_sam_map_names() was using a "blank" static string instead of
a NULL pointer for nt names.  NULL means over-ride, so the nt name
got left as "blank".

this causes nt clients to terminate with extreme prejudice.
(This used to be commit ddd350198202d6a1d2c715b3dce7db3a5d76a63a)