fix(GODT-2573): Crash on null update

Ensure that if we don't produce an update we don't construct an update
array with nil values.

internal/user/events.go

@@ -635,6 +635,10 @@ func (user *User) handleCreateMessageEvent(ctx context.Context, message proton.M
 			return nil, err
 		}
 
+		if update == nil {
+			return nil, nil
+		}
+
 		return []imap.Update{update}, nil
 	}, user.apiUserLock, user.apiAddrsLock, user.apiLabelsLock, user.updateChLock)
 }