Check Rule Silence

A proposed version of a rule, to be judged against what this organization's events do now.

rule_key null says the change creates a rule, which can only add reach.

event_types is required and is what the proposed version would be keyed to, so a check can tell an edit that keeps the event type from one that drops it. It is required because the two readings are opposite: an omitted list would say the rule is keyed to nothing any more, which reports every event it alone covered as silenced -- the loudest possible warning for a caller that merely forgot a field. Unknown types are refused the way the save that follows refuses them.

One check per previewable event type the rule is in service for.

Empty means the question does not arise -- a new rule, or a chain with no version in service -- rather than "nothing would be silenced", which is a check with silenced 0.