Popup will be without text if popupContent is empty.

[dimapaloskin]

Hi.

I use popupContentBinding in popupMixin, but in the case of if popupContent is empty
popup will be opened without text. I think it is necessary to add check in popuMixin's click event.

https://github.com/gabesmed/ember-leaflet/blob/master/packages/ember-leaflet/lib/mixin/popup.js#L15