fix: handle blank push notification config id consistently across transports

`GetTaskPushNotificationConfigParams` rejects a null `id` but permits an
empty string, and the two transports diverge on that input:

- `GrpcTransport` sends `.setId("")`.
- `RestTransport` treats it as absent and requests the trailing-slash
"default config" URL.

Identical caller code against the same `ClientTransport` method therefore
produces two different requests. This aligns both on
and adds coverage for the blank-id path, which was previously untested.

Also corrects the Javadoc on `GetTaskPushNotificationConfigParams`, which
described `id` as optional and documented `@throws` for `taskId or tenant`
when the constructor actually validates `taskId` and `id`.