fix(lang): treat an empty language selection as automatic - #4704

Summary

Passing languages=[] to language detection or metadata application raises IndexError at languages[0], before either empty-text handling or automatic detection can run. An empty selection is already treated as a default by check_language_args and by the later automatic-detection branch.

Normalize an empty language list to the existing automatic mode, just like None. This also uses the normal short-ASCII fallback. Explicit languages, the empty-string skip sentinel, and validation of non-list inputs retain their existing behavior.

Validation

• Added 18 native cases: 11 fail and 7 pass on the unchanged base.
• New and existing language tests: 83 passed, 4 skipped. The skips are optional backend tests in the existing suite.
• Covers actual langdetect output for English/French/Korean, empty/whitespace text, short-text fallback, list/iterator metadata application in whole-document and per-element modes, unchanged input lists, and existing explicit/invalid language options.
• Ruff 0.15.10 check/format and git diff --check pass. The full project test suite was not run.

Checklist

• Added regression tests and documented empty-list behavior.
• Preserved explicit language selection and invalid-input checks.
• Apply the bug label.