Mapbox hover text concatenates values if there is an empty string in `text`

[chriddyp]

http://codepen.io/chriddyp/pen/GrKLEV?editors=0010

The third marker has the hover text a,b,,d instead of nothing

first reported here: https://community.plot.ly/t/multiple-fields-in-hovertext-on-satellite-map/2999/2