Allow empty database name in OpenDBI method

[jtaczanowski]

Allow to the name value OpenDBI method as an empty string. From lmdb documentation doc name can be empty "If only a single database is needed in the environment, this value may be NULL."

[hirokiht]

The author provides opening the unnamed db as txn.OpenRoot function, but I agree the naming is not quite intuitive while we are reading the official LMDB documentation as main reference.