Make EdgeQuery return array() for empty data instead of null

Summary:
Ref T2852. When we pull edge data and an edge has none, we currently populate null in the results. This is inconvenient and makes idx()'ing it clumsy. Instead, populate array() for empty.

(We've barely used edge data anywhere so far, which is why this hasn't come up before, but I have some use cases for it now.)

Test Plan:

• Trivial / used in future diff.
• Verified existing edge data callsites don't care about this API change (there are only 3).

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6293