getPageSize used truthiness (`pageWidth ? ...`) which silently fell
through to the default for any 0-valued pageWidth/pageHeight. Aligns
with the convention getMargins already uses one block below — same
class of bug, same `!== undefined` fix.

Bumps to v0.2.15.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>