MdeModulePkg: implement NULL instance of MemoryAllocationLib library class

In order to permit MM_STANDALONE modules to be built without relying
on StandaloneMmPkg, provide a BASE type NULL implementation of
MemoryAllocationLib.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>.