Nt32Pkg/WinNtSimpleFileSystemDxe: Fix ASSERT() parsing '\'

Nt32Pkg/WinNtSimpleFileSystemDxe: Fix ASSERT() parsing '\'

https://bugzilla.tianocore.org/show_bug.cgi?id=331

If Nt32 is built using UEFI Shell from the ShellPkg sources,
an ASSERT() is generated when a single '\' character is
entered at the shell prompt.

The WinNtSimpleFileSystemDxe module GetNextFileNameToken()
function breaks a file path up into tokens, but it does not
handle the case where a FileName ends in a '\' character.
It returns an empty string instead of NULL.  The fix is
to set *FileName to NULL if the remaining file path is an
empty string.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>