Address claude-review round 4 on PR #868: tighten the assign_page_numbers
guard from `page_boundaries is not None` to a truthy check, so a PDF with an
empty boundary list no longer enters the trace span and fires the alarming
"NO page numbers assigned" warning for a harmless no-op.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>