GraphQL `DesignType` `diff_refs` should be `null: true`

The GraphQL DesignType does not allow nil values for diff_refs. If one is encountered, the GraphQL request will error.

Within the model of Design, the #diff_refs method can return nil when a Design has no Versions.

At the time of writing it is impossible for a Design to exist without there being any Versions. However, since we have code that allows diff_refs to be nil then it might be sensible to change the diff_refs property in GraphQL to be optional to avoid errors in future, if it ever becomes possible!