clang-tidy error (on not-linux): use nullptr instead of NULL

[pinheadmz]

Running clang-tidy on macos I get this error:

/Users/matthewzipkin/Desktop/work/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:84:25: error: use nullptr [modernize-use-nullptr,-warnings-as-errors]
   84 |     pthread_threadid_np(NULL, &tid);
      |                         ^~~~
      |                         nullptr

I suspect the reason this hasn't triggered failures on our CI is that this code block is not compiled on linux:

libmultiprocess/src/mp/util.cpp

Lines 80 to 90 in 22bec91

	#ifdef __linux__
	buffer << syscall(SYS_gettid);
	#elif defined(HAVE_PTHREAD_THREADID_NP)
	uint64_t tid = 0;
	pthread_threadid_np(NULL, &tid);
	buffer << tid;
	#elif defined(HAVE_PTHREAD_GETTHREADID_NP)
	buffer << pthread_getthreadid_np();
	#else
	buffer << std::this_thread::get_id();
	#endif