"crs is null" runtime error when handling a valid empty GeoServer response

[esonica]

When querying a geoserver that returns an empty collection, the following valid JSON is returned:

{"type":"FeatureCollection","totalFeatures":0,"features":[],"crs":null}

As per the GeoJSON Spec, the CRS value can be a JSON null as valid response :
http://geojson.org/geojson-spec.html#coordinate-reference-system-objects

Currently the Cesium.GeoJsonDataSource.load() function does not handle a null CRS value gracefully, throwing a runtime error.

https://github.com/AnalyticalGraphicsInc/cesium/blob/5a4b5ead78bcb6671f7a55a0cb0847d01e95383c/Source/DataSources/GeoJsonDataSource.js#L873

I don't think a runtime error is applicable when the geoJSON returned is technically valid. Can we find a better way of handling this case?

[mramato]

We could probably special-case an empty feature collection and ignore a crs value of null in this case. But empty data is the only possible use case where a crs of null "makes sense" (though why you would include it at all, I have no idea.). Do you have any other suggestions on how null crs should be handled?

Just an FYI, the official GeoJSON spec is now https://tools.ietf.org/html/rfc7946 and has actually removed the crs property altogether.

[mramato]

I fixed this with #4456 and it will go out with the next release of Cesium on November 1st.