fix: add null checks to list numbering functions

[lloyd-lighthouse]

Summary

Added null safety to the list numbering functions in shared/common/list-numbering/index.ts to prevent runtime errors when lvlText is null or undefined.

Changes

• Updated createNumbering to accept lvlText: string | null and return early if null
• Updated generateNumbering to accept lvlText: string | null with null guard
• Updated generateFromCustom to accept lvlText: string | null with null guard
• Updated return types to string | null to reflect nullable returns

Why

When lvlText is null, calling .replace() on it causes a runtime error. This fix ensures graceful handling by returning null early.

Test Plan

• Existing unit tests pass
• Verify list numbering still works correctly with valid lvlText values
• Verify null lvlText returns null without throwing errors

Breaking Changes

None - this is a backwards-compatible fix. Functions that previously crashed on null input will now return null.

[lloyd-lighthouse]

Hi team.

Its strange that a version was released to fix numbering issues recently (#1675)

However this is a similar issue, perhaps some sort of an edge case?

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This document works
broken-numbering-missing-abstract.docx

This document however, does not work
document-2026-02-02 (2).docx

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We've tried our best to trace this issue down and think the suggested fix in this PR is a way for us to make sure we never encounter this issue again, but we are obviously new to this codebase and can only do a best guess realistically.