jsonpb: with EmitDefaults=true, nil map/slice should marshal to "null"

[cybrcodr]

Current implementation with EmitDefaults=true, marshals map to JSON {} and slice to JSON [] regardless of whether map/slice is nil or empty instance. Spec does not dictate any rules on this afaik. Note that if EmitDefaults=false, empty or nil is treated as "default" value and hence is omitted in the JSON data, which I think is appropriate.

With commit 748d386, library unmarshals JSON "null" to nil map/slice instead of empty instance for the same rationale as mentioned in PR #394. So perhaps it would be more intuitive/natural to make the distinction in the reverse direction to distinguish nil vs empty, i.e. empty map marshals to {}, nil map marshals to null, empty slice marshals to [], nil slice marshals to null.

The other part that comes with the above suggestion is what to do with WKT Struct and ListValue. Struct wraps a map field "fields" and ListValue wraps a repeated field "values". However, marshaling these types are typically based on marshaling the inner fields directly if not empty/nil. If one sets a field with empty &Struct{}, the inner map "fields" is still nil. Current implementation marshals it into {} regardless of EmitDefaults.

@zombiezen -- thoughts on this?

[dsnet]

The current behavior is correct. In the protobuf data model, there is no distinction between an "unpopulated" list or map and an "empty" list or map. Therefore, a pure protobuf reflection implementation of JSON cannot distinguish between these two possible outputs.