[Maps] Setting Polygon fill color to transparent is not applied on map

[nickpeihl]

Kibana version:
7.6, master

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

Describe the bug:
When a polygon geo_shape is added to Elastic Maps as a Document layer and the Polygon fill color text box is erased, the text box value changes to "Transparent". However, the Map still retains the original fill color.

Steps to reproduce:

1. Add a Documents layer containing geo_shape polygons to Elastic Maps (for example, use the GeoJSON Uploader for the USA States GeoJSON
2. Delete the Contents of the "Fill color" box in the Layer Style settings.
3. Note that the Fill color text box says "Transparent", but the polygons on the map are still filled in.

[elasticmachine]

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