Added null safety to the Drupal gallery migration

[anirudhprabhakaran3]

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Ref: #11185

[gitpod-io]

[anirudhprabhakaran3]

Adds null safety to #11371

[codeclimate]

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[github-actions]

This pull request generated screenshots of many common pages in the running app. You should be able to download and view them here:
https://github.com/publiclab/plots2/suites/8141885241/artifacts/353064051