ShellPkg/Ls: Return empty content for all empty folders

ShellPkg/Ls: Return empty content for all empty folders

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2296

Currently, when 'ls' is run on an entirely empty directory (this
includes not having '.' and '..'), the output is always 'File not
found'. For when not filtering its children, this patch rather
displays the usual header and footer.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Marvin Haeuser <mhaeuser@outlook.de>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>