Leave boolean fields null unless a value is in the column

[MBARIMike]

This will make the appearance of the Patch_test and Repeat_survey columns better by have a '-' unless it's checked with an 'x', 'yes', or now anything in the spreadsheet. Removed values in the production database with:

Mission.objects.exclude(patch_test__isnull=True).update(patch_test=None)
Mission.objects.exclude(repeat_survey__isnull=True).update(repeat_survey=None)

Applies to #206