fix(code-block): preserve empty lines in code blocks

[Jiasm]

Description

Fix empty lines collapsing in code blocks by inserting a newline character for empty rows. Previously, empty lines had no content causing the <span> elements to collapse to zero height, affecting both visual display and copy behavior.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Related to empty line rendering in code blocks

Changes Made

• Detect empty rows in code block rendering (row.length === 0 or single token with empty content)
• Insert "\n" for empty lines instead of rendering an empty span
• Ensures empty lines maintain proper height visually
• Ensures copied text includes empty lines correctly

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

Tested with code containing empty lines to verify:

1. Empty lines display with correct height
2. Copy/paste preserves empty lines

Screenshots/Demos

Before: Empty lines collapse to zero height
After: Empty lines maintain proper line height and copy correctly

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

This is a minimal fix that inserts "\n" for empty lines. Alternative approaches considered:

• CSS min-height: 1lh - only fixes visual, doesn't preserve copy behavior
• Zero-width space \u200B - fixes visual but copies as invisible character, not newline

[vercel]

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