A fill extrusion layer doesn't follow color opacity when using rgba for fill-extrusion-color paint property?

Nobody has claimed this yet.

**mapbox-gl-js version**:
v1.12.0

**browser**:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36
https://www.whatsmybrowser.org/b/QZK8A

### Steps to Trigger Behavior

Example that shows behavior:

1. Visit: https://jsbin.com/feyulic/edit?html,outputl
2. When the "county-extruded" layer is not visible initially by setting fill-extrusion-opacity: 0, the map displays as expected. The three counties matched in the getColor() function are correctly filled red, green, and blue. The unmatched counties are colored with 'rgba(204, 204, 204, 0)'. Because the rgba alpha opacity is set to 0 on the unmatched counties, they correctly do not fill. The lightblue background color on #map-container shows through as expected.

.

3. When the 'Show extruded layer' button is clicked and the county-extruded layer is shown by setting fill-extrusion-opacity to 1, the "other" counties don't seem to follow the 0 opacity for fill-extrusion-color. They appear colored in black.

.

This may be related. https://github.com/mapbox/mapbox-gl-style-spec/issues/556

I am experimenting with dynamically styling a vector layer. Setting color of a fill type layer looked promising, but when I extended the idea to a fill-extrusion layer, I ran into this issue.

### Expected Behavior
fill-extrusion-color should follow the alpha channel as fill-color does.

### Actual Behavior
fill-extrusion-color fills black if the alpha channel is 0

Start with the JSBin reproduction linked in the issue and compare the fill and fill-extrusion rendering paths when fill-extrusion-opacity is changed from 0 to 1. The fix is complete when an rgba fill-extrusion-color with alpha 0 remains transparent instead of rendering as black, while the reported colored counties still render correctly.

Written by the indexing model from the issue text.