[openstreetmap/openstreetmap-website] Indicate if the message preview is empty (Issue #3748)

[danieldegroot2]

The message preview should indicate to the user it is empty, similar to how GitHub says "Nothing to preview". It should either
- Display such text on the preview; or
- grey out the preview button. (may have hover text or similar to indicate the reason)

**Current behaviour** (large empty area)
![image](https://user-images.githubusercontent.com/67521919/194766966-9742040c-ceef-41ab-8de4-2181a9e3473c.png)


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