Make RankingScoreThreshold optional

[danFbach]

Pull Request

Related issue

Fixes #601

What does this PR do?

• make RankingScoreThreshold nullable (thereby optional and ignored by json convertor if left null)

PR checklist

Please check if your PR fulfills the following requirements:

• Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• Have you read the contributing guidelines?
• Have you made sure that the title is accurate and descriptive of the changes?

Passes all current integration tests.

[curquiza]

bors merge

[meili-bors]

Build succeeded:

• integration-tests
• format-check