[IMP] account: bank account journals form: hide related fields when bank_account_id isn't set

Showing these fields does not actually make sense when bank_account_id isn't set as well, since setting them and saving will result in them losing their value (as they're actually related on an empty field). We need to hide them while bank_account_id is not set.

closes odoo/odoo#48842

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>