This wrapper calls json.loads() but also handles None (returning None),
which enables the code at many call sites to be simplified.

Removed some callers' `if isinstance(field, str): ...` code, which has
the effect of newly disallowing field values that are already dicts.
However we've verified that all *.from_db() calls have raw database
outputs as their arguments, so such fields will be always be strings
and IMHO giving a dict to from_db_json() is really a logic error that
should be detected.

In SequencingGroupInternal.from_db() added `pop(..., None)` so that
a missing meta field is now accepted. The previous code suggests that
having pop() produce KeyError here was unintended.

The expected argument types for from_db_json() are listed in the
definition, but we don't list its return type. The best we could
say in general is `object` but most call sites expect `dict[str, str]`
(or occasionally `list[str]`) due to the shape of their expected JSON.
Specifying `object` would lead to mypy errors at these call sites.

Expands test coverage to include audit_log.py from_db_json change.

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