fix: prevent writing to nonexistent firestore docs

Using the same livekit project means we may receive events for another
environment, though we should eventually separate the environments.

go/internal/handler/handler.go

@@ -1132,7 +1132,7 @@ func (h *Handler) HandleLivekitWebhook(w http.ResponseWriter, r *http.Request) {
 
 	_, err = docRef.Update(ctx, []firestore.Update{
 		{Path: "huddle_active_participants", Value: participantIds},
-	})
+	}, firestore.Exists)
 	if err != nil {
 		slog.Error("failed to update huddle participants in firestore", "error", err, "path", docPath)
 	}