Windows: Fix directory to service null mask lookups

The direct to service for non-wildcard lookups, commit
b7ba97ad537bd0e9a241f052ddd1c3a50c74745b, introduced a bug when the search
mask is left unspecified.   Do not set bNonWildcardMatch to TRUE
when the mask is NULL.

Change-Id: I6c4846b443acc7e5e42d4e83e75ef383fc400db9
Reviewed-on: http://gerrit.openafs.org/9542
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>