Investigate empty awareness messages

[max-nextcloud]

We are seeing some empty awareness messages being send to the clients which led to errors in the browser. We are now filtering them out on the server - but they should not exist in the first place.

I don't know if these are expected off the top of my head. I think we still store the awareness messages in the session data. That might cause an empty string if no awareness has ever been send from the users but the session still exists.

Would be good to clean up the awareness handling.

Originally posted by @max-nextcloud in https://github.com/nextcloud/text/issues/6937#issuecomment-2726322710