Fix deleted_tasks field type in TaskDeletion struct

[coinmoles]

Pull Request

Related issue

Fixes #665

What does this PR do?

• Change the type of deleted_tasks field in TaskDeletion from usize to Option<usize>
  • This should be Option<usize> since it can be null according to the API documentation

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• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
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