ShellPkg/Ls: Handle the case when SearchString is NULL
ShellPkg/Ls: Handle the case when SearchString is NULL

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <9fa3a6894a36bca79e6a72ad65af5a5702efecd1@intel.com>
Reviewed-by: Hao Wu <f387b71200577e27ccf5abb090fda0ddd187ec6e@intel.com>

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