Fix migration if Whisperfish was started but never registered

If the old Whisperfish was ever only started but not registered, only the storage directory exists. This is because the db directory was only created after registration, when the SQLite database was actually created. This situation confused the migration process. The solution is to simply create the old db directory, if the only directory that exists is the old storage.

Tested on Xperia 10 II where beta.11 aborted with false false false true folders, with this patch the migration was successful:

$ harbour-whisperfish
No old database found, creating empty directory...
Migrating old db folder...
Migrating old storage folder...
Storage folders migrated

Fixes #450 (closed)