Tweak treatment of falsy values to capture checkbox value behavior better.

As far as I could tell in testing, this did not seem to change other field behavior (and of course, doesn't cause unit tests to fail), but I'd definitely like someone to think through this and possible give me some other ideas for testing and possible edge cases and so on. Thanks!
Unless you were calling `Fieldmanager_Context_Post ::save_fields_for_post()`