don't crash on nullptr pdfOptionsPage

Summary:
same as in fbeff5ae7c2b27577d829839879b0966b5a3859a there may not be
a pdfOptionsPage when using print preview.

this fixes an exciting crash on our windows build when trying to print
preview

Test Plan: can't easily build on windows but the traceback definitely falls over 0x0000 on that line, so given context I am confident this fixes the crash

Reviewers: aacid

Reviewed By: aacid

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D24696