Handle empty plots
Handle empty plots

Don't add the GeoJSON data if the plot is empty. Also set a default
map view when there's no data so the map just displays like a regular
slippy map.

Addresses the behavior reported in #12

zeapo/mplleaflet,zeapo/mplleaflet,BibMartin/mplleaflet,jwass/mplleaflet,ocefpaf/mplleaflet,jwass/mplleaflet,ocefpaf/mplleaflet,BibMartin/mplleaflet