Fix handling of "null" domain values in Charts

Summary: Depends on D20487. If you min(1, 2, null), you get null. We want 1.

Test Plan: Viewed a "burnup for project X" chart where one dataseries had no datapoints. Saw a sensible domain selected automatically.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Differential Revision: https://secure.phabricator.com/D20488