swap_using_offset: Enhance with skipping empty data

[nordicjm]

An original part of the swap using offset mode was skipping copying sectors that were empty, this had to be omitted as it was flagging errors in the simulator tests (likely because on revert the sections that did have data were not erased). It would be good to revisit this and fix it properly as it would prevent erasing/swapping/reverting sectors that are empty