ShellPkg/rm: fix hang when deleting an absolutely-empty directory

ShellPkg/rm: fix hang when deleting an absolutely-empty directory
An ordinary empty directory should contain "." and ".." entries.
When an empty directory even doesn't contain "." or ".." entry,
FileHandleFindFirstFile() may return error status and a NULL
FileInfo.
IsDirectoryEmpty() implementation in Rm.c doesn't consider this
case and the deference of FileInfo->FileName causes page fault
exception because FileInfo is NULL.
The patch checks the return status of FileHandleFindFirstFile()
to fix this issue.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>