Add default value to some FEC fields
JOURNALCODE, PIECEREF and ECRITURELIB are required to have a value.
For the JOURNALCODE, we could just use the journal name as fallback.
For the two others, there are not always a value for them, so I propose to use the same trick as Odoo [1] and fill it with '-' instead of empty string.
[1] https://github.com/odoo/odoo/blob/10.0/addons/l10n_fr_fec/wizard/account_fr_fec.py#L139