title: If data source doesn't emit data for a while, does the watermark of Flink operator keep advancing?
body: <p><a href="https://i.stack.imgur.com/5exyr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5exyr.png" alt="watermark" /></a></p>
  <p>Just curious, if the data source doesn't emit data for a while, whether Flink operator still receive watermark or not? the <code>Low Watermark</code> in the image will keep advancing or stay still?</p>