[Maps] Cannot select Solid fill-color when removing fields

[thomasneirynck]

The style-editor gets in an unusable state with this interaction pattern:

Steps:

• add a vector tile layer
• add a field in the source-settings
• do by-value styling on that field for fill-color
• remove that field from the source-settings

-> The "by value/solid" drop-down is disabled

[elasticmachine]

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

[nreese]

VectorStyle.getDescriptorWithMissingStylePropsRemoved needs to be updated to convert dynamic style to static style when source updates result in no fields. I am working on a fix