[Lens] [Color mapping] Color mapping applied to additional layers does not work if first layer is empty

[gvnmagni]

I know this is a very peculiar case and please feel free to give this issue the right amount of interest, but I found this weird little bug in Lens.

Basically, if the first layer is empty (no Y axis, no X axis, no breakdown) within the second layer the Color Mapping feature does not work, I believe that the list of data series is not recognised for some reason (they don't appear in the dropdown menu when you start digit their name and the automatic mapping doesn't work, as if it deosn't find any data series to be matched with a color).

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)