fix(backup): Fix OrganizationMember.user_id type confusion bug

[azaslavsky]

The code previously assumed that user_id from OrganizationMember could not be None. The gradual typing for python is unable to deduce that what we assumed was set[int] is actually, in practice, set[int | None] because of this. There is now an explicit check-and-remove for this in the constructor for Filter, which relies on this invariant.