fix(subgraph): Conditionally print @specifiedBy directive on scalars correctly

[angelo-moreira]

When having a schema with scalars

scalar Date
scalar JSON
scalar HexColour

We end up in a situation where the scalar is undefined instead of null.

Because of that when we try to get the sdl we receive the @specifiedBy tag with an object attached to it.

query getIntrospection {
  _service {
    sdl
  }
}

This PR should fix that by weakly comparing undefined == null, I am happy to do a more strict comparison.

[apollo-cla]

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[trevor-scheer]

Thanks @angelo-moreira, this LGTM.

• I confirmed the test was previously failing before the change
• This is how the graphql-js printer does it (which this is a fork of) https://github.com/graphql/graphql-js/blob/47bd8c8897c72d3efc17ecb1599a95cee6bac5e8/src/utilities/printSchema.ts#L296

Would you please add a CHANGELOG entry for this in subgraph-js/CHANGELOG.md?

I'll get this cherry-picked / backported to the version-0.x branch.

[angelo-moreira]

Thank you for the quick reply @trevor-scheer, I have now added a line to the changelog with hopefully a useful commit, please do let me know if you need me to do something else, it's my first time contributing to this project.

[trevor-scheer]

A great first contribution @angelo-moreira! I made a few minor tweaks to save you the trouble, but otherwise this is excellent. Thanks again!