title: Bounds are not valid error calling fitBounds
body: <p>I have Nominatim GeoJSON data representing a country's border such as this <a href="https://nominatim.openstreetmap.org/search?country=it&amp;polygon_geojson=1&amp;format=json" rel="nofollow noreferrer">https://nominatim.openstreetmap.org/search?country=it&amp;polygon_geojson=1&amp;format=json</a> and getting it through ajax. I'm using Leaflet to create and display the map.</p>
  <p>After calling <code>geoJson(jsonData)</code> I want the map to fit this border with <code>fitBounds()</code> but I get <code>Uncaught Error: Bounds are not valid.</code>.</p>
  <p>This is my code:</p>
  <pre><code>$.ajax({ url: NOMINATIM_BOUNDARY_URL })
  .done( function(json) {
    console.log(json); // the json is loaded correctly.
    var layer = L.geoJson(json);
    console.log(layer.getBounds()); // this prints 'undefined'. Why?
    mapref.fitBounds(layer.getBounds()); // mapref is our map object.
  });
  </code></pre>
  <p>I think I'm getting the error because bounds are undefined. Why?</p>
  <p>Thanks.</p>