merge: no way to fallback on another value than current if current is empty

In case of 3-way merge that involves an empty value for current, the result will be always an empty value, even if the fallback configuration stated that it should fallback on previous or next version
Moreover the problem is not logged as a conflict, the only log error being "The current value is empty". Note that this is not the case if it's next that is empty.
We should have the same mechanism if it's next or current that is empty, and this should be logged and processed as a conflict.