Apply clang-tidy modernize-use-nullptr to googletest.

Now that googletest has moved to C++11, it should no longer
use NULL or 0 for the null pointer. This patch converts all
such usages to nullptr using clang-tidy.

This prevents LLVM from issuing -Wzero-as-null-pointer-constant
warnings.

PiperOrigin-RevId: 215814400

googlemock/test/gmock-more-actions_test.cc

@@ -333,7 +333,8 @@ TEST(InvokeTest, FunctionWithUnusedParameters) {
 
   Action<int(int, int, bool, int*)> a2 =
       Invoke(SumOfFirst2);
-  EXPECT_EQ(23, a2.Perform(make_tuple(20, 3, true, static_cast<int*>(NULL))));
+  EXPECT_EQ(23,
+            a2.Perform(make_tuple(20, 3, true, static_cast<int*>(nullptr))));
 }
 
 // Tests using Invoke() with methods with parameters declared as Unused.