Fix issues show api to return empty geojson correctly

Author: sanak

app/views/issues/show.api.rsb

@@ -26,7 +26,7 @@ api.issue do
   if @issue.geom
     api.geojson (params[:format] == "json") ? @issue.geojson : @issue.geojson.to_json
   else
-    api.geojson ""
+    api.geojson nil
   end
 
   render_api_custom_values @issue.visible_custom_field_values, api