Empty messages should not be possible to send

[SystemKeeper]

How to use GitHub

• Please use the 👍 reaction to show that you are affected by the same issue.
• Please don't comment if you have no relevant information to add. It's just extra noise for everyone subscribed to this issue.
• Subscribe to receive notifications on status change and new comments.

---

Steps to reproduce

1. Open a conversation
2. Press spacebar
3. Press return

Expected behaviour

Nothing should happen, because the message is empty (after trim).

Actual behaviour

The message is actually send as an empty message. This happens since switching to NCRichContentEditable (I think). Corresponding issue of the vue-lib nextcloud-libraries/nextcloud-vue#4068.
Not sure if we should prevent this on frontend or backend.

Talk app

Master, stable26

[nickvergessen]

The backend and frontend should prevent this