Update @Nullable/@NonNull annotations

[nikita-tkachenko-datadog]

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What does this PR do?

A cosmetic improvement: some @Nullable and @NonNull annotations are updated to better reflect whether certain methods can or cannot return null values.
This helps to understand the code better and avoid NullPointerException.

Description of the Change

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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