fix: TypeError when document.collaboratorIds is null (#12471)

Guard against null collaboratorIds when persisting collaborative
updates; the DB column has no default and can be NULL on older rows.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

server/commands/documentCollaborativeUpdater.ts

@@ -71,7 +71,7 @@ export default async function documentCollaborativeUpdater({
     const pud = new Y.PermanentUserData(ydoc);
     const pudIds = Array.from(pud.clients.values());
     const collaboratorIds = uniq([
-      ...document.collaboratorIds,
+      ...(document.collaboratorIds ?? []),
       ...sessionCollaboratorIds,
       ...pudIds,
     ]);