Created by: jakesen
This provides a solution for issues like these: https://github.com/thoughtbot/administrate/issues/441
The solution is provided by allowing each field class to decide how to handle empty values via a normalize class method. A substitute value can be specified with an empty_value option when initializing a field using with_options.
This PR should probably have a little more test coverage, but first I want to see what adjustments might need to be made to the implementation to make it acceptable.