don't crash on nullptr pdfOptionsPage

Summary:
same as in fbeff5ae 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
1 job for Applications/19.08 in 14 minutes and 13 seconds
Status Name Job ID Coverage
  Build
passed linux stable-kf5-qt5 qt5.12 #19665

00:14:13

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