[Alerting] event log for action execution writes space_id instead of namespace

[pmuellr]

In the event log for action execution, we're currently writing the space_id of a namespace into the document instead of the namespace (namespace == undefined yields space_id default). Specifically, we're writing namespace: "default" for the default space, but instead shouldn't be writing any value, or sending namespace: undefined (should be able to do either).

The code writing generating that document is here:

kibana/x-pack/plugins/actions/server/lib/action_executor.ts

Lines 113 to 116 in f9ba963

	const event: IEvent = {
	event: { action: EVENT_LOG_ACTIONS.execute },
	kibana: { namespace, saved_objects: [{ type: 'action', id: actionId }] },
	};

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)