start method Null safety

Implementation

Future start() async {
  if (size == 0 || _isRunning) return;

  _isRunning = true;
  _emitEvent(QueueEventType.queueStart);

  while (size > 0) {
    if (_isForcedClosed) break;
    await _dequeue();
  }

  _isRunning = false;
  _emitEvent(QueueEventType.queueEnd);
}