title: how openlayer show lower resolution tiles at high zoom level
body: <p>Here's my situation：i have worldwide tiles level 0 to 7, and some key area tiles level bigger than 7.</p>
  <pre><code>map.addLayer(
              new ol.layer.Tile({
                  source: new ol.source.XYZ({
                      minZoom: 0,
                      maxZoom: 18,
                      url: url
                  })
              })
          )
  </code></pre>
  <p>the problem is, when the zoom level bigger than 7 at a key area, it shows like this:
  <img src="https://ftp.bmp.ovh/imgs/2020/12/9ebcb41842b479a0.png" alt="" />
  what I expect is when the zoom level bigger than 7 at a key area, other areas can show lower resolution, not blank.
  what should I do this right?</p>