fix: preserve blank lines with body length limit

[schlotter]

Description

Fix body_length_limit so it preserves intentional blank lines in commit bodies while still wrapping non-empty lines to the configured width.

Checklist

• I have read the contributing guidelines

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Generated-by: GitHub Copilot CLI following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes — not required, just a fix

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly — no documentation changes
• Check and fix any broken links (internal or external) — no documentation changes

Expected Behavior

With body_length_limit enabled, Commitizen should preserve blank lines in the commit body and only wrap long non-empty lines.

Steps to Test This Pull Request

1. Configure body_length_limit = 80 or run cz commit --body-length-limit 80.

2. Create a commit message body with a paragraph break, for example:

   First paragraph line
   
   Second paragraph line
   

3. Confirm the generated commit message still contains the blank separator line.

4. Run uv run pytest tests/commands/test_commit_command.py -k body_length_limit.

5. Run uv run ruff check commitizen/commands/commit.py tests/commands/test_commit_command.py.

6. Run uv run ruff format --check commitizen/commands/commit.py tests/commands/test_commit_command.py.

Additional Context

This fixes a regression where textwrap.wrap("") caused empty body lines to be dropped when body_length_limit was enabled (#2079).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (d914b4d) to head (171a234).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2080      +/-   ##
==========================================
+ Coverage   98.19%   98.26%   +0.07%     
==========================================
  Files          61       61              
  Lines        2829     2829              
==========================================
+ Hits         2778     2780       +2     
+ Misses         51       49       -2     

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[Manny7717]

Review — Approved

Verified locally on head: 6/6 tests pass (body-length-limit and message-length-limit params, including the new preserves_blank_lines case).

The fix is correct and minimal: textwrap.wrap returns [] for empty input, which is why blank lines were silently dropped; short-circuiting with [line] preserves paragraph breaks exactly as intended.

One optional, non-blocking suggestion (inline): the if line check only catches fully-empty strings — a whitespace-only body line still gets dropped because textwrap.wrap(' ') is []. if line.strip() else [line] would preserve those too, if the intent is to preserve any intentional blank-looking line.