message: dispatcher: fix comment.
diff: @@ -91,7 +91,7 @@ private:
  void runPostCallbacks();
  // Validate that an operation is thread safe, i.e. it's invoked on the same thread that the
  - // dispatcher run loop is executing on. We allow run_tid_ == nullptr for tests where we don't
  + // dispatcher run loop is executing on. We allow run_tid_ to be empty for tests where we don't
  // invoke run().
  bool isThreadSafe() const {
  return run_tid_.isEmpty() || run_tid_ == api_.threadFactory().currentThreadId();
  