Fix GeoJSON.initialize when no data

[jmvivo]

When you try to create a L.Proj.GeoJSON layer with no data (to add it
after layer creation):

    var layer = L.Proj.geoJson();
    layer.addData(geojson).addTo(map);

You get an exception:

    Uncaught TypeError: Cannot read property 'features' of undefined

[edrex]

I hit this problem right off, and this pull request fixes it.

It's pretty common to create a GeoJSON layer synchronously and then add the data later, so this bug is critical for GeoJSON support.

The code change looks fine.

👍

[pthorin]

Thanks!