[backport#14268] Make SafeDbt DB_DBT_MALLOC on default initialization

Summary:
If we're constructing the SafeDbt without provided data, it is always malloced,
so that is the case we expose.

Also run clang-format.

https://github.com/bitcoin/bitcoin/pull/14268/commits/4a86a0acd9ac3ca392f0584a5fd079a856e5e4ba#

Depends on D7322

Concludes backport of Core PR14268

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7323