s3/auth map NULL domains to our global sam name

s3/auth map NULL domains to our global sam name

This is an addendum to d8c54fdd, which made make_user_info_map() match
Windows behavior by mapping untrusted domains given to smbd on the wire
with the users credentials to smbd's global sam name.

This fix was being circumvented in the case where the client passed
a NULL domain.  Vista clients do this.  In that case smbd was always
remapping the name to the machine workgroup.  The NULL domain case
should also be mapped to the global sam name.

Removing the code in this patch, causes us to fall down to the logic
added in d8c54fdd and properly map the domain.