[Maps] Dynamic icons only work when the field is also driving another style-property

[thomasneirynck]

To reproduce:

1. Create new vector layer with logs sample data
2. Create dynamic fill color on agent.keyword
3. Create dynamic icon on agent.keyword -> this works
4. Select new field for dynamic icon: geo.dest -> this does not work. Icons no longer display
5. Select new field for dynamic color: geo.dest -> now it works again. Icons display correctly
6. Change dynamic color to solid color: -> icons break again

An error is being logged to the console on breakage:

:5601/dhb/9007199254740991/bundles/maps.bundle.js:30347 Uncaught TypeError: Cannot read property 'name' of null
    at onFieldChange (:5601/dhb/9007199254740991/bundles/maps.bundle.js:30347)
    at onFieldChange (:5601/dhb/9007199254740991/bundles/maps.bundle.js:28252)
    at EuiComboBox.onRemoveOption (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.@elastic.js:67562)
    at EuiComboBox.removeLastOption (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.@elastic.js:67292)
    at :5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.@elastic.js:67477
    at HTMLUnknownElement.callCallback (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:306777)
    at Object.invokeGuardedCallbackDev (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:306826)
    at invokeGuardedCallback (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:306881)
    at invokeGuardedCallbackAndCatchFirstError (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:306895)
    at executeDispatch (:5601/dhb/9007199254740991/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:307025)

[elasticmachine]

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