CIDEVMI-1421 Fix empty unprocessed data bug in sio mule common drivers

@wfrench Please review.
If there was no unprocessed data remaining in the driver state, this was not being handled properly by the driver the next time it tried to use that state.  This has been corrected, and example tests added to dosta (to test the single file sio mule common dataset driver) and flort (to test the multiple harvester sio mule common dataset driver).