win32: use nullStdin instead of /dev/null

Summary:
On Windows, `/dev/null` is meaningless, use the `nullStdin` method instead.

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: genevievehelsel

Differential Revision: D51056121

fbshipit-source-id: 08502767a25178d77ed98152a1cc6539c7007d64

watchman/main.cpp

@@ -354,7 +354,7 @@ static SpawnResult spawn_win32(const std::vector<std::string>& daemon_argv) {
 
   ChildProcess::Options opts;
   opts.setFlags(POSIX_SPAWN_SETPGROUP);
-  opts.open(STDIN_FILENO, "/dev/null", O_RDONLY, 0666);
+  opts.nullStdin();
   opts.open(
       STDOUT_FILENO,
       logging::log_name.c_str(),