Ignore empty or relative XDG_CONFIG_HOME values

[OskarEichler]

Summary

XDG_CONFIG_HOME='' currently produces /app, and relative settings produce relative application paths. Ignore empty and relative values and use the existing HOME/.config fallback.

Reproduction and verification

Checked unset/empty/dot/relative/tilde settings, absolute root, spaces and Unicode paths. The XDG Base Directory Specification requires absolute paths and defines the empty/unset fallback.

• External focused repro corpus: 9 checks pass against this isolated branch, under rbenv Ruby 4.0.6.
• Existing upstream suite: 36 examples, zero failures before and after the patch (RBENV_VERSION=4.0.6 rbenv exec rspec -Ilib -Ispec).
• Source-only change; no existing tests modified or new test files added, per the contributor's task constraint. The focused corpus lives outside the repository.
• Local verification was on macOS. Other Ruby versions and native operating systems were not executed. Upstream RuboCop configuration cannot load its legacy rubocop-lts1_8.yml, including after installing current rubocop-lts; lint status is not represented as passing.

Breaking-change notes

No API removal. Invalid environment settings now fall back to HOME/.config; valid absolute paths and existing Windows/macOS branches retain their behavior.