Accept null in the issueSetDueDate mutation

What does this MR do?

See #327287 (closed)

Allows the issueSetDueDate GraphQL mutation to accept null:

mutation SetDate {
  issueSetDueDate(input:{ projectPath: "jashkenas/underscore", iid: "5", dueDate: null }) {
    issue {
      iid
      title
      dueDate
    }
  }
}

This removes the dueDate from the issue.

Screenshots (strongly suggested)

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