fix: handle empty string with errors='replace'

[santhreal]

converter._convert_nopinyin_chars hits TypeError when ord empty string errors=replace. This PR fixes the regression with a focused test covering the case.

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of empty text when using replacement or ignore modes.
  • Empty input now consistently returns an empty result without triggering errors.

• Tests

  • Added coverage to verify expected behavior for empty input across supported error-handling modes.

[coderabbitai]

📝 Walkthrough

Walkthrough

The converter treats empty replacement input like other non-single-character inputs, avoiding ord() conversion. Tests verify that empty strings return empty lists for default, replace, and ignore error modes.

Changes

Empty input handling

Layer / File(s)	Summary
Handle and test empty input pypinyin/converter.py, tests/test_pinyin.py	The replacement branch now includes empty input, and tests cover empty strings with replace and ignore error modes.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main fix: handling empty strings with errors='replace'.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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