Mark questionable column nullability for later

Summary:
Ref T1191. Ref T6203. While generating expected schemata, I ran into these columns which seem to have sketchy nullability.

• Mark most of them for later resolution (T6203). They work fine today and don't need to block T1191. Changing them can break the application, so we can't autofix them.
• Forgive a couple of them that are sort-of reasonable or going to get wiped out.

Test Plan: Saw 94 remaining warnings.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: hach-que, epriestley

Maniphest Tasks: T1191, T6203

Differential Revision: https://secure.phabricator.com/D10593