[NFC] Fix PHP 7.4 errors on ContributionPage tests

[seamuslee001]

Overview

This fixes 2 test failures when running the unit tests on PHP 7.4

Before

Tests fail

          <error type="PHPUnit\Framework\Error\Notice">CRM_Contribute_Form_Contribution_ConfirmTest::testPayNowPayment
Trying to access array offset on value of type null

/home/jenkins/bknix-edge/build/build-2/web/sites/all/modules/civicrm/CRM/Contribute/Form/Contribution/Confirm.php:2718


          <error type="PHPUnit\Framework\Error\Notice">CRM_Contribute_Form_Contribution_ThankYouTest::testLiveAndTestContributionStatus
Trying to access array offset on value of type null

/home/jenkins/bknix-edge/build/build-2/web/sites/all/modules/civicrm/CRM/Contribute/Form/Contribution/ThankYou.php:183

After

Tests pass

ping @eileenmcnaughton

[civibot]

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[seamuslee001]

Test fail unrelated