fix: omit null fields when publishing TrafficInfluence SRM commands

        if self._publisher is None:

            raise RuntimeError("DataBus publisher is not available")

        try:

            await self._publisher.publish(subject, command.model_dump(mode="json"))

            # srm/canonical-parameters-schema.md: "prefer omitting inapplicable keys; do

            # not send null placeholders unless a capability explicitly defines null

            # semantics" -- none of TI's optional fields (domain_id, traffic filter

            # protocol, deactivate's external_ref/service_instance_id) do.

            await self._publisher.publish(

                subject, command.model_dump(mode="json", exclude_none=True)

            )

        except Exception as exc:

            raise DownstreamServiceException(error_msg) from exc