R1:c77bfd660374

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…