make sure DB is initialized not just existing

[fkooman]

the sqlite database is automatically initialized on first run, so no need to do that manually, but if that fails for some reason the first time a 0 byte sqlite file is written which will be used as an indicator that the DB is already initialized.

So, do not rely only on the fact that the sqlite file exists that the DB is initialized, be more clever about it.

Fix for #57.