Null message fields included in wire JSON instead of omitted (RSL1e, RTL6i3)

[paddybyers]

Spec points

• RSL1e: "If any of the values are null, then key is not sent to Ably."
• RTL6i3: Same requirement — a payload with null `data` should be sent as `{ "name": "click" }`, omitting the `data` key entirely.

Observed behaviour

When `data` is `null`/`undefined`, ably-js includes it as `"data": null` in the JSON wire format instead of omitting the key. Same for `name`.

Message serialization in `src/common/lib/types/message.ts` does not strip null/undefined values before `JSON.stringify`.

Failing tests (2)

Test	Expected	Actual
`RSL1e - null name omitted from body`	Body has no `name` key	Body has `"name": null`
`RSL1e - null data omitted from body`	Body has no `data` key	Body has `"data": null`

Reproduction

```bash
RUN_DEVIATIONS=1 npx mocha --grep "RSL1e - null" test/uts/rest/channel/publish.test.ts
```

From PR #2191, branch `uts-rest`.

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