explicit nullable and allow !! in testclasses

[midttuna]

// explicit nullable and allow !! in testclasses

Checklist

The following aspects have been respected by the author of this pull request, confirmed by both pull request assignee and reviewer:

• Adherence to coding conventions
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• Adherence to javadoc conventions
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• Changelog update (necessity checked and entry added or not added respectively)
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• README update (necessity checked and entry added or not added respectively)
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• config update (necessity checked and entry added or not added respectively)
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• SDCcc executable ran against a test device (if necessary)
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