Whitespace-only bookmark title renders as a blank row in --list, inconsistent with the written filename

[adisakshya]

Context

split_book.py, print_plan() vs slugify() / _walk_outline().

Problem

_walk_outline only falls back a bookmark's title to "Untitled" when the title is empty (item.title if item.title else "Untitled"); a title that's present but whitespace-only (e.g. " ") passes that check untouched. print_plan then prints that title as-is, producing a blank-looking row in the --list preview table. Later, when the file is actually written, slugify() strips the whitespace down to nothing and falls back to "untitled" for the filename. So the on-screen plan (blank) and the actual output filename (..._untitled.pdf) disagree — confusing right at the point (--list) users are meant to be able to trust before committing to a real split.

Reproduction

1. Build a PDF with an outline bookmark titled " " (three spaces) alongside a normal chapter.
2. python split_book.py that.pdf --level 1 --list — the row for that entry shows a blank title.
3. python split_book.py that.pdf --level 1 (real run) — the written file is named ..._untitled.pdf.
   Confirmed with a synthetic PDF.

Expected

The --list preview and the actual written filename should agree — e.g. treat a whitespace-only title the same as an empty one in _walk_outline (.strip() before the truthiness check), so the plan table also shows Untitled.

Actual

Preview shows a blank title; actual filename uses the untitled fallback. Cosmetic but confusing.

Acceptance criteria

• A whitespace-only bookmark title displays consistently (e.g. as Untitled) in both --list output and the written filename.
• Regression test added.