ShellPkg: Expand special output file to include "NULL" and case insensitive

ShellPkg: Expand special output file to include "NULL" and case insensitive

As per ECR 1349 change in UEFI Shell Specification 2.2, expanding
a special output file name to include "NULL". Previously it only
supported "NUL" as a special output file and it was case sensitive.
With this change both "NUL" and "NULL" are special output file and
checked as case insensitive.

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