walletdb: Ensure that having no database handle is a failure
walletdb: Ensure that having no database handle is a failure

Previously having no database handle could still be considered a success
when BerkeleyDatabase and BerkeleyBatch were used for dummy database
things. With dedicated DummyDatabase and DummyBatch classes now, these
should fail.


Former-commit-id: 980e0eebd6f7ea893d4a14a4d1e41872d85cd9b2
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin