Support Optional Dropdown field for additional domain fields
How to make a dropdown list field `Optional`:
a) support empty-valued options
b) fix the value comparision to use strict operator `===`
If you leave out `Default` or configure it to empty string, the pre-selection of the default option might fail.
It fails in case you have multiple option values that are decided to be truthy (https://stackoverflow.com/a/2382510)
In that case every such option would be pre-selected (`selected` in DOM) and thus finally the last selected option will appear as pre-selected.