[Bug]: toCoreMessage defaults optional taskId to "" instead of undefined

### What happened?

When a v0.3 message does not have a `taskId`, `toCoreMessage` in the compat layer sets it to `""` instead of leaving it as `undefined`:

```
// dist/compat/v0_3/client/index.js, line 996
taskId: compatMsg.taskId ?? "",
```

`taskId` is optional in v0.3, so not every message has one. Setting it to `""` causes problems for consumers that check whether `taskId` is present, because `""` looks like a value but is not a valid task ID. In JavaScript, an optional field that is not set should be `undefined`, not `""`.

### Relevant log output

```shell

```

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