Allow "0" to be a valid workboard column point limit

Summary:
Fixes T6580. Now:

• Empty field means "unlimited".
• Zero means 0.
• Nonzero means that number.

(Although you can now have fractional points, I didn't change columns to allow fractional limits, because too bad.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6580

Differential Revision: https://secure.phabricator.com/D15239