fix(epub): optimize spine membership lookup and avoid leading blank lines on missing metadata

[Chau Vu (chauvuusvn)]

Description

This PR addresses two improvements in EpubConverter:

1. O(1) Set Membership Lookup: In EpubConverter.convert(), the loop over spine called if file in z.namelist(): on every chapter iteration, repeatedly reconstructing the archive entry list ((N \times M)$). We now utilize the precomputed zip_names set ((1)$).
2. Avoid Leading Blank Lines when Metadata is Missing: When an EPUB document lacks metadata tags (dc:title, dc:creator, etc.), metadata_markdown is empty, but markdown_content.insert(0, "\n".join(metadata_markdown)) still inserted an empty string into index 0, producing leading blank lines in the final markdown output. We now only prepend metadata when non-empty.
3. Filter Empty Chapter Items: Ensures empty converted spine items don't accumulate excessive line breaks.

Testing

• Added regression test test_epub_without_metadata_has_no_leading_blank_lines() in packages/markitdown/tests/test_epub_converter.py.
• All 5/5 tests in test_epub_converter.py pass.