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>

SecurityPkg/Library/VariableKeyLibNull/VariableKeyLibNull.inf

@@ -0,0 +1,33 @@
+## @file
+#  Provides Null version of VariableKeyLib for build only.
+#
+#  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010029
+  BASE_NAME                      = VariableKeyLibNull
+  FILE_GUID                      = 2B640ED8-1E6A-4516-9F1D-25910E59BC4A
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = VariableKeyLib
+
+#
+# The following information is for reference only and not required by the build tools.
+#
+#  VALID_ARCHITECTURES           = IA32 X64 Arm AArch64
+#
+
+[Sources]
+  VariableKeyLibNull.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  SecurityPkg/SecurityPkg.dec
+
+[LibraryClasses]
+  BaseLib
+  DebugLib
+