title: Python Bokeh HoverTool avoiding display NaN
body: <p>I have used GeoJSONDataSource (converted from a geopandas) to display ALL the world map in my bokeh figure.</p>
  <pre><code>geosourceword = GeoJSONDataSource(geojson = json_data)
  standardfig.patches('xs','ys', source = geosourceword,fill_color = {'field':'deaths', transform':color_mapper})
  </code></pre>
  <p>Then I use</p>
  <pre><code>hover_tool = HoverTool(tooltips = [ ('location', '@location'), ('# of deaths', '@deaths')])
  </code></pre>
  <p>to display over a location the number of deaths. Since, in my data, some country have deaths = NaN I don't want to display them (for instance in this picture I don't want to display Turkey). Any idea how I can proceed ?
  <a href="https://i.stack.imgur.com/XvrD7.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XvrD7.jpg" alt="enter image description here" /></a></p>