OvmfPkg/IntelTdx: Add TdxHelperLibNull

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4243

TdxHelperLib provides below helper functions for a td-guest.
 - TdxHelperProcessTdHob
 - TdxHelperMeasureTdHob
 - TdxHelperMeasureCfvImage
 - TdxHelperBuildGuidHobForTdxMeasurement

TdxHelperLibNull is the NULL instance of TdxHelperLib.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/IntelTdx/TdxHelperLib/TdxHelperLibNull.inf

@@ -0,0 +1,32 @@
+## @file
+#  TdxHelperLib NULL instance
+#
+#  Copyright (c) 2021 - 2023, Intel Corporation. All rights reserved.<BR>
+#
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = TdxHelperLibNull
+  FILE_GUID                      = 853603b2-53ea-463d-93e6-35d09a79e358
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = TdxHelperLib
+
+#
+# The following information is for reference only and not required by the build tools.
+#
+#  VALID_ARCHITECTURES           = X64
+#
+
+[Sources]
+  TdxHelperNull.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[LibraryClasses]
+  BaseLib