[CBM] backup resume fails due to NULL collection_id

We saw in a CV run an unexpected error when resuming a backup:

failed to get active chunks stats: sql: Scan error on column index 0, name "collection_id": converting NULL to uint32 is unsupported 

collection_id is taken from a uint32 Go struct field should it shouldn't be possible for us to have inserted a NULL here. The process was killed so it may be a corrupted database