Carbon API is definitely rolling 12 month period only

Carbon backfill API fetch can only truly be for the past 12 months from today - it is definitely only a rolling 12 month period.

So rather than intentionally generating an error for any carbon export backfill request past twelve month, simply assume no data and force the writing of an empty data file (if required).

This is a low priority - the carbon export backfill works as it is.