Handle empty streamable array

[coorasse]

This PR originates from a difficult debugging session I had.

In my model I had a typo:

after_create_commit -> { broadcast_append_to @chatroom }

and used @chatroom instead of chatroom. This error lead to an empty streamables array ([nil]) and the broadcasting was not working.

The error symptoms were:

• the message does not appear on screen
• I look at browsers network log and I don't see any message arriving in the websocket
• I look at the logs and I spot the following:

[ActionCable] Broadcasting to : "<turbo-stream action=\"append\" ...

where you can see that the final stream name ends up being simply :. I had therefore an hard time to spot the issue.

This PR is very aggressive, and simply raises in such a context, maybe we would like a lighter (error message) approach? I think that if we have an empty streamables array is most probably an error.