ShellPkg:?cd \? command fails to go back to the root directory of a file system

Allows cd command to go back to the root directory when 'cd \' executed in system.

This change prevents last PathRemoveLastItem() call which truncates '\' from 'fs0:\'
in desired root path which is required to set CWD to the root directory.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Tapan Shah <tapandshah@hpe.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>