Allow null as an acceptable value for issue weight in GraphQL API

What does this MR do?

See #334338 (closed)

Allows the issueSetWeight GraphQL mutation to accept null:

mutation SetWeight {
  issueSetWeight(input:{ projectPath: "jashkenas/underscore", iid: "5", weight: null }) {
    issue {
      iid
      title
      weight
    }
  }
}

This removes the weight from the issue.

Screenshots (strongly suggested)

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