MdePkg/BaseCacheMaintenanceLibNull: Add Null instance for host testing

MdePkg/BaseCacheMaintenanceLibNull: Add Null instance for host testing

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2799

The services in CacheMaintenanceLib usually generate exceptions in a
unit test host application.  Provide a Null instance that can be safely
used.

This Null instance can also be used as a template for implementing
new instances of CacheMaintenanceLib.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>