bundleFor method
Returns the open bundle if initialize has been called for
atSign and the bundle is not closed; otherwise null.
Useful for code paths that should be no-ops when persistence
is not yet wired (e.g. metrics on a not-yet-started secondary).
Implementation
@override
AtPersistenceBundle? bundleFor(String atSign) {
final b = _bundles[atSign];
return (b == null || b.isClosed) ? null : b;
}