walletdb: Ensure that having no database handle is a failure

Summary:
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.

This is a backport of core#19102 [3/3]
https://github.com/bitcoin/bitcoin/pull/19102/commits/0fcff547d5b47822c13104978fda0c486e596526

Depends on D10049

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10050