payroll_l10n_fr: fix for DSN sorts

in ec835d86 (payroll_l10n_fr: add missing sorts in DSN report, 2024-02-28)
we added sorts so that the report is deterministic when hash randomization
is enabled, but this was not enough, when iterating through employee_ctp
to find the social_contribution_organisation, not all records have a
corporate_registration_code, so we check that it's present before
accessing it.

Also, one `sorted` that was added was incorrect, because the elements
are dict, so they can not be compared on python3.