fix(common): ignore relative XDG directory overrides

[unbridled-41]

Summary

Ignore relative values in XDG directory environment variables and use the existing fallback paths.

Root cause

The XDG base directory specification requires these environment variables to contain absolute paths. The current helpers accepted any non-empty value, so relative configuration could make runtime, cache, or config paths depend on the process working directory.

Changes

• Use XDG_RUNTIME_DIR, XDG_CACHE_HOME, and XDG_CONFIG_HOME only when absolute.
• Preserve the existing fallback behavior for unset, empty, or relative values.
• Add regression tests for relative and valid absolute overrides.

Test plan

• Release test target build
• Four focused XDGTest regression cases
• git diff --check

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: unbridled-41

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[deepin-ci-robot]

Hi @unbridled-41. Thanks for your PR.

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