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