fix: Handle TOC items with uid=None to prevent TypeError when writing EPUB

[yusunglee2074]

Summary

• Some EPUB files have TOC (Table of Contents) items where the uid attribute is None
• When ebooklib tries to write the NCX file, it passes this None value to lxml's SubElement which expects bytes or unicode, causing a TypeError
• This fix adds a _fix_toc_uids() method that recursively processes the TOC and assigns auto-generated uids (navpoint-0, navpoint-1, etc.) to any items that have uid=None

Error before fix

File "ebooklib/epub.py", line 1356, in _create_section
    np = etree.SubElement(itm, "navPoint", {"id": item.uid})
TypeError: Argument must be bytes or unicode, got 'NoneType'

Test plan

• Tested with an EPUB file that has uid=None in TOC items
• Verified that the generated EPUB file is valid and can be opened

🤖 Generated with Claude Code

[yihong0618]

nice