Allow runningWorker / renderChild handlers to return null and avoid re-render.

[rjrjr]

The only way a handler can no-op is by returning noAction() or equivalent, which means every time any worker or child workflow fires a render pass must happen. We've talked about noticing when state and output are not changed, and avoiding re-render at that point, but defining "changed" is daunting, and it's very late in the game for such a semantic shift. But allowing an explicit null return should be easy and safe.

Should deprecate noAction() at the same time.

[rjrjr]

I wonder if we can go deeper in this direction. E.g., can we instrument WorkflowAction.Updater.state such that if state = is not called; and the parent consuming setOutput returns a null action; we short circuit in that case too.

That's riskier. Existing code might be inadvertently relying on rendering direct changes to mutable state, or to external state. But that seems like a pretty narrow space.

cc @steve-the-edwards, WDYT?

[steve-the-edwards]

Quick note so we don't lose context on this. @jamieQ and I paired on this and given currently how WorkerWorkflow routes output via the actionSink even having an output handler for runningWorker that could return null wouldn't avoid a re-render as we need to execute that Output handler deeper in internal code for WorkerWorkflow in order to prevent an action from hitting the sink (and therefore re-rendering).

[steve-the-edwards]

Not needed due to solution for #985

[steve-the-edwards]

Covered by #985 and fixed by #992