[NFC] Fix test failures on PHP 7.4 caused by either NULL being set fo…

[seamuslee001]

…r the membership_type_id or Payment Processor not being set on the form

Overview

This fixes 2 test failures on PHP 7.4 as per title

Before

Tests fail

          <error type="PHPUnit\Framework\Error\Notice">CRM_Event_Form_ParticipantTest::testSubmitUnpaidPriceChangeWhileStillPending
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/AbstractEditPayment.php:695

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

/home/jenkins/bknix-edge/build/build-2/web/sites/all/modules/civicrm/CRM/Member/Form.php:500

After

Tests pass

ping @eileenmcnaughton

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).