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. Alice must be not a participant.
• Be user Bob and send a message in that Room.

Now Alice does not see a crash anymore, but the message from Bob.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15497

Differential Revision: https://we.phorge.it/D25408