skip over empty geometries (Issue #117)

Umap may create exports with empty geometries, containing

  "coordinates": []

Mapnik does not consider those geometries as valid, so we
now test for these and skip over them when generating the
converted GEOjson output for rendering

ocitysmap/stylelib/Umap.py

@@ -247,6 +247,8 @@ class UmapStylesheet(Stylesheet):
 
             # now go over the actual geometry features in that layer
             for feature in layer['features']:
+                if not feature['geometry']['coordinates']:
+                    continue
                 try:
                     # feature properties override previous defaults
                     new_props = get_default_properties(feature, layer_defaults)