MdePkg: add null version of RngLib

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1871

This is null version of RngLib which should be used with modules that
inherit an (indirect) dependency on the RngLib class, but never actually
call RngLib APIs for consuming randomness.

To be more specific, if following components or functionalities are used
in a platform, the BaseRngLibNull should *not* be used. Instead, a non-Null
version of RngLib must be used (like BaseRngLib for IA32/X64, or future
DxeRngLibRngProtocol for all ARCHs).

- HddPasswordDxe.inf
- AES, TLS (TlsDxe.inf, TlsLib.inf), RSA_OAEP, RSA_PK1
- (If BaseRngLibNull interface ASSERTed at boot time)

Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/BaseRngLibNull/BaseRngLibNull.uni

@@ -0,0 +1,14 @@
+// /** @file
+// Null Instance of RNG (Random Number Generator) Library.
+//
+// Copyright (c) 2019, Intel Corporation. All rights reserved.<BR>
+//
+// SPDX-License-Identifier: BSD-2-Clause-Patent
+//
+// **/
+
+
+#string STR_MODULE_ABSTRACT             #language en-US "Null Instance of RNG Library"
+
+#string STR_MODULE_DESCRIPTION          #language en-US "This library instance should be used with modules that inherit an (indirect) dependency on the RngLib class, but never actually call RngLib APIs for consuming randomness."
+