don't crash on nullptr pdfOptionsPage
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
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
Reviewed By: aacid
Differential Revision: https://phabricator.kde.org/D24696