Make checkbox states boolean

              I didn't realize this earlier either, but the airtable -> bq pipeline interprets checkbox states as `true/null` not `true/false`. We can post-process that away at some point, but for now this should work.

_Originally posted by @niharikasingh in https://github.com/georgetown-cset/eto-st-newsfeed/pull/173#discussion_r1224329378_
            