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-generated-function-mockers_test.cc

@@ -228,7 +228,7 @@ TEST_F(FunctionMockerTest, MocksDecimalFunction) {
                                  Lt(100), 5U, NULL, "hi"))
       .WillOnce(Return(5));
 
-  EXPECT_EQ(5, foo_->Decimal(true, 'a', 0, 0, 1, 0, 0, 5, NULL, "hi"));
+  EXPECT_EQ(5, foo_->Decimal(true, 'a', 0, 0, 1, 0, 0, 5, nullptr, "hi"));
 }
 
 // Tests mocking a function that takes a non-const reference.