SecurityPkg: add null version of VariableKeyLib

SecurityPkg: add null version of VariableKeyLib

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2594

Add null version of VariableKeyLib instance. The full version should be
provided by platforms which supports key generator.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Nishant C Mistry <nishant.c.mistry@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>