ci: fall back to GITHUB_REF_NAME when GITHUB_BASE_REF is empty

[Oreoxmt]

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[gemini-code-assist]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-08 02:47:02.63171438 +0000 UTC m=+408695.505064352: ☑️ agreed by lilin90.

[Oreoxmt]

/approve

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