Update NetLibGetMacAddress() to handle the case correctly when SNP is absent.

Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ni, Ruiyu <ruiyu.ni@intel.com>
Reviewed-by: Jin, Eric <eric.jin@intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@14904 6f19259b-4bc3-4df7-8a09-765794883524