Don't remove empty structs.

-- in.cue --
deployment: [ID=string]: {
}

deployment: alertmanager: {
	empty: {}
	volumes: [{
		name: "alertmanager"
		emptyDir: {}
	}]
}
-- out/trim --
== in.cue
deployment: [ID=string]: {}

deployment: alertmanager: {
	empty: {}
	volumes: [{
		name: "alertmanager"
		emptyDir: {}
	}]
}
