fix: omit protobuf data for events without payload

[immanuwell]

Fixes #1193

protobuf marshal always sets data, even when the event has no payload.
after a protobuf round trip that becomes DataEncoded=[]byte{}, then JSON marshal emits busted JSON:

{"specversion":"1.0","id":"id","source":"source","type":"type","datacontenttype":"application/json","data":}

Repro on main:

ev := ce.NewEvent(ce.VersionV1)
ev.SetID("id")
ev.SetSource("source")
ev.SetType("type")
ev.SetDataContentType(ce.ApplicationJSON)

b, _ := format.Protobuf.Marshal(&ev)
var out ce.Event
_ = format.Protobuf.Unmarshal(b, &out)
_, _ = out.MarshalJSON()

This patch skips protobuf data when e.Data() == nil, so no payload stays no payload. small fix, but pretty real.

Tests:

• add a regression test for protobuf -> event -> json round trip with no data
• go test ./... in binding/format/protobuf/v2
• go test ./... in v2

[duglin]

@immanuwell thanks. Fix seems ok but I have a question, why would json Marshal() ever return bad json like that? It doesn't seem possible to me even if the data were nil

[duglin]

Since we can't control all of the messages we might receive it feels like we might want to also add protection against receiving these nil port but messages, no?

[kunalworldwide]

Good fix. Without this, ToProto always sets Data to a BinaryData wrapper even when the event has no payload. That empty BinaryData{BinaryData: nil} gets serialized as a zero-length bytes field in protobuf, which on the Unmarshal side creates an empty []byte{} instead of nil — and when that's subsequently marshalled to JSON, you get "data_base64":"" instead of no data field at all, producing invalid JSON for events like heartbeats that intentionally have no payload.

The guard on DataContentType == ContentTypeProtobuf is also correct — no point trying to wrap nil data into an Any message.

Test is thorough: roundtrips through protobuf marshal/unmarshal, verifies DataEncoded is nil, then confirms the JSON output is clean and re-parseable.

LGTM.