Fix missing keys acquisition_era_id and processing_era_id for some DBS2 data

Hi @YuyiGuo,

this patch fixes the problem with missing acquisition_era_id and processing_era_id for datasets from DBS 2, which do not have any aquisition_era and processing_era and are requested to migrate between two DBS 3 instances.

Please, review the code and merge.

Thanks.
Manuel