[Maps] Layer legend color remains black when grouping by strings

[n0othing]

Kibana version: 7.7

Describe the bug:

A map layer's legend color remains black when using a custom color ramp + grouping by strings instead of numbers

Steps to reproduce:

1. Add Sample web logs sample data in Kibana.
2. Open the [Logs] Total Requests and Bytes and create a layer using the parameters shown in the screenshot.
3. The new layer's legend icon will be black.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

The problem exists in https://github.com/elastic/kibana/blob/master/x-pack/plugins/maps/public/classes/styles/vector/components/legend/extract_color_from_style_property.js#L28. The logic pulls the first color from the color ramp but is pulling customColorPalette[0].colorCategory instead of customColorPalette[0].color.

I will put up a fix