[REF] Fix PDF Test failure on php versions 7.4 and later

[seamuslee001]

Overview

This fixes the following test failure that is present in 5.43 and master for php versions 7.4 and later

CRM_Contribute_Form_Task_PDFTest::testSendPDF
Trying to access array offset on value of type null

/home/jenkins/bknix-max/build/build-2/web/sites/all/modules/civicrm/CRM/Contribute/Form/Task/PDF.php:183

Before

Test fails on php7.4 and later

After

Test should pass on php7.4 and later

ping @eileenmcnaughton @colemanw

[civibot]

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[eileenmcnaughton]

It's a bit alarming that this change is needed tbh - but I guess until we get CI running on php 7.4 it's just gonna be this sort of stuff happening & it gets tests passing