UnitTestFrameworkPkg/GoogleTestLib: Enhance unit test NULL checks

gtest.h uses standard includes that define NULL to __null. This
is different that the C++ standard that uses nullptr. Redefine
NULL in GoogleTestLib.h to use C++ form of nullptr. This allows
NULL to be used in checks and mocks without explicit typecasts.

Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>

UnitTestFrameworkPkg/Include/Library/GoogleTestLib.h

@@ -13,6 +13,15 @@
 #include <gmock/gmock.h>
 #include <cstring>
 
+//
+// For all use of GoogleTestLib, make sure NULL is defined to nullptr to
+// support matching any unit test pointer value to NULL.
+//
+#ifdef NULL
+  #undef NULL
+#define NULL  nullptr
+#endif
+
 using ::testing::Throws;
 using ::testing::ThrowsMessage;
 using ::testing::HasSubstr;