walletdb: Add BerkeleyDatabase::Open dummy function

Summary:
Adds an Open function for the class abstraction that does nothing for
now.

This is a backport of core#19334 [3/4]
https://github.com/bitcoin/bitcoin/pull/19334/commits/2179dbcbcd0b9bef7ad9c907b85294b9a1bccf0f

Depends on D10023

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10024