[NFC] Fix E-notice in Afform unit tests

[seamuslee001]

Overview

Fixes the following e-notice in Afform unit tests

api_v4_AfformFileUploadTest::testSubmitFile
Trying to access array offset on value of type null

/home/jenkins/bknix-max/build/build-0/web/sites/all/modules/civicrm/ext/afform/core/Civi/Api4/Action/Afform/Get.php:59

Before

E-notice exists in tests on php7.4 and up

After

No E-notice

ping @colemanw

[civibot]

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[colemanw]

Looks good, thanks @seamuslee001