Fix null test plan database error

Summary:
Some Differential fields are not nullable; when Test Plan is switched to non-required mode we can end up trying to save a null value to a non-nullable column (see D2193).

(I should probably just alter the schema to make these fields nullable, but that might have farther-reaching effects.)

Test Plan: Reproduced error, applied patch, no more error.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2200