Show chat message with meeting_user set to NULL

[r-peschke]

Describe the bug
If the (meeting-)user will be deleted meeting_user_id will be set to NULL in the chat message. In consequence there is no user to show

How to Reproduce
Steps to reproduce the behavior:
see OpenSlides/openslides-backend#2566

Expected behavior
Show a replacement text like "user removed" (the problem could be reproduced by deleting a user or by deleting a meeting_user instance)

Additional context
In current main the backend throws an error, if you try to delete/remove a user with a chat message.
With PR OpenSlides/openslides-backend#2571 from backend and OpenSlides/openslides-meta#148 from meta you'll get the chat message withount meeting_user_id.