Optional fields: clearing an optional input makes it mandatory

Error:

29119982 (HEAD -> master, zehiro/fix-873, upstream/master) Ignore (or delete if previously set) unset optional fields. Fixes #873 (closed).

STR:

1. On first access creation (at least), select "Caisse d'épargne" bank
2. Fill anything in mandatory fields
3. In "user code" type anything
4. Empty the "user code" field
5. Submit the form