Valideer dat mensen met automatische incasso een IBAN hebben

One-sentence description

A member can now be set to be paying by direct debit, but not have a valid IBAN.

Current behaviour

A member can now be set to be paying by direct debit, but not have a valid IBAN. This creates empty records in !503 (merged).

Expected behaviour

Members should only be allowed to pay by direct debit if they have a valid IBAN.

Steps to reproduce

1. Set a member's IBAN to the empty string
2. Edit a member to say that they pay by direct debit
3. Observe that this is possible