R1:8507d3a95072

Fix Exception in Chat room when you are not a Participant

Summary:
After this change you can lurk in public chats, and receive notifications, without crashing your planet.

Fix exception "Undefined index: PHID-USER-..." shown to newcomers when they are receiving messages
from a public Chat they are not members of.

Closes T15497

Test Plan:
- Have the Notification server (Aphlict) enabled and running.
https://we.phorge.it/book/phorge/article/notifications/
- Be user Alice and visit a Conpherence Room.