l10n_it_delivery_note: error in sequence name generation

[francesco-ooops]

Keep generating name with sequence number until one is available.

This could be useful because if for any weird reason a DN with the next sequence has already been created it would show a validation error.

• [ ] 14.0 https://github.com/OCA/l10n-italy/pull/4052
• [ ] 16.0