[Maps] Color legend not showing in "Custom color ramp"

[crisdarocha]

Kibana version: 7.4.2

Elasticsearch version: 7.4.2

Server OS version: ESS

Browser version: Chrome 78.0.3904.97

Browser OS version: MacOS 10.14.6

Original install method (e.g. download page, yum, from source, etc.): ESS

Describe the bug: When adding a choropleth layer and choosing a Custom color ramp, the legend is not shown (see screenshot below).

Steps to reproduce:

1. Follow the instructions for "Adding a choropleth layer"
2. At the layer style step, instead of choosing a gradient color, choose Customer color ramp
3. After completed, go to the legends and the custom color levels are not displayed.

Expected behavior: See value ranges for each color.

Screenshots (if relevant):

Errors in browser console (if relevant): None

Provide logs and/or server output (if relevant): None

Any additional context: None

[elasticmachine]

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