fix JS error when enabling hexlayer using layer control
fix JS error when enabling hexlayer using layer control

This looks like a bug in leaflet-d3: in HexbinLayer.onRemove() the _data
member is set to null, but I think it should be set to [] instead.
This is now worked around by manually setting some empty data after hiding
the layer.

oliver/pyffheatmap,oliver/pyffheatmap,oliver/pyffheatmap