Fix nullable query param

[bw-flagship]

Fixes #24736

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@jaumard (2018/09) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

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Summary by cubic

Makes dart-dio clients omit optional query parameters when the provided value is null. Previously, optional nullable params were emitted as key=null; now guards follow requiredness: optional params are omitted when null, while required params (including nullable) are always sent. Fixes #24736.

• Apply an if (param != null) guard to all non-required query params in api.mustache.
• Add an OpenAPI 3.1 fixture and a regression test covering optional vs. required (including nullable) behavior.
• No change to encoding or required parameter handling; regenerate clients.

^{Written for commit f95c2e5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}