[Lens] support dynamic coloring for missing values

[dej611]

Describe the feature:
Most visualization types only represent the buckets from Elasticsearch. However, by nature of their grid-based visual structure, heatmap and datatable interpolate cells where there is no data from Elasticsearch. These cells aren't null values, but rather data that isn't there.

Because these missing data cells have no associated data, they aren't considered when applying formulas or dynamic coloring.

In the screenshot below, the user expected the missing cells to be colored green. Since they are empty, they are "zero" values.

I think it would make sense to let the user decide how to treat missing values.
As @mbondyra pointed out operations like count have a special behaviour for missing values which may show confusing results when combined with dynamic coloring (some cells would be colored because set at 0, while others won't because -).

We could interpret missing values as zero, or introduce a separate color setting.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)