[Maps] Custom vector tile layer can not be styled by value when field is number

[nickpeihl]

Kibana version:
7.9.0-snapshot

Describe the bug:
A 3rd party vector tile source containing a number field can not be styled by value of that number field. However, a string field can be styled by value.

Steps to reproduce:

1. Add a pbf vector tile source (https://tiles.maps.elastic.co/data/v3/{z}/{x}/{y}.pbf?license=6c33988f-587e-4415-bf80-805f964c50fd)
2. Set the Tile layer to mountain_peak
3. Add the name and class fields as type string and the ele field as type number.
4. Change the Fill color to By value and choose the ele field.
5. Add values such as 0, 655, 1310, 1965, 2620, etc.
6. Notice that the fill color is hollow.

Expected behavior:
The fill color should be assigned the corresponding range value. This only happens with fields of type number.

Screenshots (if relevant):

[elasticmachine]

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