Talked to @kamil about this check. Here's what he had to say:

Hey, I was thinking about this a bit and my first idea was to put this check here, after generating messages and then return all IDs of generated messages as failed. However, looking deeper, when there is no viewerID we not generate messages anyway so I think we can just return empty array here and should be fine

On the other hand, it will be strange case to send composable messages when there is no viewer so I think invariant could work too

I decided to go with an empty array because I'd rather avoid crashing the app with an invariant. Ultimately it's probably a scenario that we never suspect to occur