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