make includeDeprecated non nullable

[martinbonnin]

• graphql-js PR: Make includeDeprecated non-null graphql-js#4354

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In the same spirit as #1040, make includeDeprecated non-nullable in the introspection schema.

I don't think there is ever a reason to passe includeDeprecated: null?

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[benjie]

This is technically a breaking change if anyone did explicitly pass null to this field (unlikely):

{
  __schema {
    queryType {
      fields(includeDeprecated: null) {
        name
      }
    }
  }
}

but it's not a breaking change for existing queries that use nullable variables:

query I($includeDeprecated: Boolean) {
  __schema {
    queryType {
      fields(includeDeprecated: $includeDeprecated) {
        name
      }
    }
  }
}

unless of course you explicitly pass the variable set to null (unlikely).

One hesitation: if there are applications that use a query like the one above and use a system where null and not set are not differentiated (such as Facebook's system), then it's possible that they are passing null explicitly as the variable value. We should try and find out if such use-cases exist before accepting this.