title: Add option for Blank TileLayer in Leaflet LayerGroup
body: <p>I have a collection of BaseMaps I want users to be able to select:</p>
  
  <pre><code>var BaseMaps = {
      // TODO blank tileLayer
  
      "Default": L.tileLayer('https://{s}.tiles.mapbox.com/v3/{id}/{z}/{x}/{y}.png', {
          id: 'examples.map-20v6611k',
          noWrap: true
      }),
  
      "ESRI Roads": L.tileLayer('http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', {
          noWrap: true
      })
  };
  </code></pre>
  
  <p>These layers are added to the LayerGroup control of my map with <code>L.control.layers(BaseMaps).addTo(map)</code> and work as expected</p>
  
  <p>The problem is I <strong>want</strong> the ability for the user to select a blank tile layer from the same control, effectively turning off the base map, but a null or blank tileLayer via <code>L.tileLayer()</code> does not work as this breaks Leaflet functionality</p>
  
  <p>Is there a clean way to add a blank tileLayer option to this control? Digging through the API and various github/forum questions has not surfaced anything.</p>
  
  <p>Thanks in advance!</p>