R1:10afe1f2b59c

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