What actually appears to happen is that sup-dump starts up and creates an empty
xapian index (because I have no xapian index) and then proceeds to generate a
dump file for that which would be empty.

Whilst this was user error, its not obvious. Also if a user follows the xapian
upgrade instructions from the mailing list (sup-dump/restore from dump etc) and
doesnt check their dump file they run the risk of losing info. Should sup refuse
to run if there is a ferret index present rather than ignoring it? At least a
user would then have a clue what they have to do.

sup-dump produces an empty dump file with git next (0.11 is probably closest
release). I updated this morning after the release announcement and started the
process to switch to xapian. Luckily I checked the dumpfile produced because it
turned out empty :(