Elide null macros key for unit test overrides

[CallumFriend-MP]

Fixes #16009.

dbt parse --use-v2-parser fails with FusionParserSchemaError on any project with a unit test that sets overrides: without macros:, because the parser emits "macros": null and Python's UnitTestOverrides.macros is a non-Optional Dict.

UnitTestOverrides.macros is Verbatim<Option<T>>, not a bare Option<T>. #[skip_serializing_none] matches on the declared outer type, so it does not apply through the wrapper, and Verbatim's Serialize forwards to the inner value. Its siblings env_vars and vars are bare Option<T> and elide correctly. Introduced by 7c17262, which wrapped the field and landed between the a5 and b1 parser releases; 1.12.1 and 1.12.2 raise the parser floor to >=2.0.0b1, so the current release pulls the failing pair by default.

This adds an explicit skip_serializing_if to that one field. Verbatim derefs to T, so the predicate is a one-liner and the wrapper 7c17262 needs is untouched.

Verification

I could not build the monorepo here, so I compiled the exact patched struct in isolation against dbt-yaml 0.9.6 with this repo's dependency versions, including the DbtSchema derive and #[skip_serializing_none]:

macros unset : {"vars":{"some_var":"'x'"}}
macros set   : {"macros":{"run_started_at":"2024-01-01"},"vars":{"some_var":"'x'"}}
round-trip   : macros.is_none()=true vars=true

Line 1 is the fix. Line 2 confirms a populated macros map still serialises, so the datetime behaviour 7c17262 was added for is preserved. Line 3 confirms the elided form deserialises back to None. Before the change the same struct emits {"macros":null,"vars":{"some_var":"'x'"}}, reproducing the reported failure byte for byte. #16009 has a repro needing no warehouse connection.

I have not run dbt-core's own suite. Understood that fork CI needs ci:approve-public-fork-ci and that it clears on each push.

Not included

A Python-side coercion of None to {} in UnitTestOverrides would also fix it, cover vars and env_vars, and unlike this change would help projects already pinned to a released b-series parser. It belongs on 1.latest rather than here, so I have left it out and noted it on the issue.

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