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

src/applications/conpherence/controller/ConpherenceUpdateController.php

@@ -328,7 +328,12 @@ final class ConpherenceUpdateController
       ->executeOne();
 
     $non_update = false;
-    $participant = $conpherence->getParticipant($user->getPHID());
+
+    // The User is always available. The Participant may not. See:
+    // User:        it's you, lurking the Chat (maybe it's a public chat).
+    // Participant: it's you, if you are a Chat Member.
+    // https://we.phorge.it/T15497
+    $participant = $conpherence->getParticipantIfExists($user->getPHID());
 
     if ($need_transactions && $conpherence->getTransactions()) {
       $data = ConpherenceTransactionRenderer::renderTransactions(
@@ -336,7 +341,7 @@ final class ConpherenceUpdateController
         $conpherence);
       $key = PhabricatorConpherenceColumnMinimizeSetting::SETTINGKEY;
       $minimized = $user->getUserSetting($key);
-      if (!$minimized) {
+      if (!$minimized && $participant) {
         $participant->markUpToDate($conpherence);
       }
     } else if ($need_transactions) {