Improve rendering of empty workboard columns in header views

Summary:
Depends on D20271. Ref T10333. When a column is empty but a board is grouped (by priority, owner, etc) render the headers properly.

When a column has headers, don't apply the "empty" style even if it has no cards. This style just makes some empty space so you can drag-and-drop more easily, but headers do the same thing.

Test Plan: {F6264611}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10333

Differential Revision: https://secure.phabricator.com/D20272