Sections: When mapping webapp fields to a section, add the ability to map nothing to a field

By default, it currently maps the name, which is fine as a default, but if the section has logic to hide things if blank, field mapping should allow to map no value to the field so it remains blank if nothing fits.

E.g. if a blogs section expects an author, but the webapp mapping to it has no author (or this date is not directly in the table), allow the user to set that mapping to empty and the section should recieve null data.