ShellPkg: Fix the incorrect return status in function FindFiles()

ShellPkg: Fix the incorrect return status in function FindFiles()

According to the latest shell spec, in function FindFiles(),
when no files were found, it should return EFI_NOT_FOUND.
But current codes don't follow the spec.
This patch is to fix this issue.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>