Emit empty array in JsonPB if EmitDefaults is enabled

[zackangelo]

Forces an empty-valued array field to be serialized if EmitDefaults is set to true.

Mirrors Java's protobuf to JSON implementation when includingDefaultValueFields is enabled.

[zackangelo]

@dsymonds @zombiezen nudge :)

[zombiezen]

LGTM from a casual glance. @bcmills, thoughts?

[bcmills]

Unless there are conflicts with the proto3 spec, the behavior of jsonpb should arguably mimic the standard json package. (EmitDefaults: false should correspond to omitempty.)

By my reading of the spec, the non-EmitDefaults behavior is currently out-of-spec:

The default value for repeated fields is empty (generally an empty list in the appropriate language).
[...]
If a field has the default value in the protocol buffer, it will be omitted in the JSON-encoded data by default to save space.

We do not appear to omit empty non-nil slices when EmitDefaults is false, but we probably should. (Perhaps that's a separate issue?)

That doesn't give us much guidance for nil slices when EmitDefaults is true, though. I would argue that we should follow the json package and emit a null in this case rather than an empty list:
https://play.golang.org/p/r6LBqu4mLI

Per the proto3 spec, "null is accepted as the empty list []", and that would remove another inconsistency between jsonpb and json.

[hyangah]

I agree with @bcmills on emitting null for an empty list. Any objection?

@zackangelo can you update your PR and also add a test?

[bcmills]

@zackangelo Are you still working on this? Is the PR ready for another round of reviews?

[cybrcodr]

I just reviewed and merged in 7a211bc. I didn't see this PR beforehand and wasn't aware that there's a preference for null over [] from this discussion. Note that [] uses less characters than null. Deserialization should handle either case anyways. Let me know if there is any strong objection to that.