[NFC] Fix APIv4 Conformance tests on php8

[seamuslee001]

Overview

This aims to fix the following test failure when the conformance tests are run on php8

api\v4\Entity\ConformanceTest::testConformance with data set "ACL" ('ACL')
Trying to access array offset on value of type null

/home/jenkins/bknix-max/build/build-0/web/sites/all/modules/civicrm/CRM/Member/BAO/MembershipType.php:393
/home/jenkins/bknix-max/build/build-0/web/sites/all/modules/civicrm/CRM/Member/BAO/Membership.php:286

Before

Conformance tests fail on php8 as no MembershipType is present in the database

After

Tests pass

ping @eileenmcnaughton

[civibot]

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