Preparations for [email protected]. (apollographql/apollo-server#3712)

* Adjust test to not use deprecated `introspectionQuery` constant.

Instead, use `getIntrospectionQuery` instead, which has been around since
before [email protected], thus within our supported version ranges.

* Adjust documentation links for moved `introspectionQuery` page.

This file now lives at `getIntrospectionQuery`.

* Applying upstream modification to `printDescription`.

I'm assuming we'll want to keep this change which was also applied upstream,
though I'm not sure what our longer term plans are for keeping up with these
changes.

Ref: https://github.com/graphql/graphql-js/pull/2177/files#diff-71ba52e9c625f826d2b0df2963c8633aR320

* Remove empty descriptions which will be included in SDL w/graphql@15.

In `graphql@15`, empty descriptions are intentionally included in the SDL
output.  In order to be excluded entirely, they must be absent (or `null`).

Ref: graphql/graphql-js#2177

Apollo-Orig-Commit-AS: apollographql/apollo-server@131c9b8

federation-js/src/directives.ts

@@ -23,48 +23,40 @@ import {
 
 export const KeyDirective = new GraphQLDirective({
   name: 'key',
-  description: '',
   locations: [DirectiveLocation.OBJECT, DirectiveLocation.INTERFACE],
   args: {
     fields: {
       type: GraphQLNonNull(GraphQLString),
-      description: '',
     },
   },
 });
 
 export const ExtendsDirective = new GraphQLDirective({
   name: 'extends',
-  description: '',
   locations: [DirectiveLocation.OBJECT, DirectiveLocation.INTERFACE],
 });
 
 export const ExternalDirective = new GraphQLDirective({
   name: 'external',
-  description: '',
   locations: [DirectiveLocation.OBJECT, DirectiveLocation.FIELD_DEFINITION],
 });
 
 export const RequiresDirective = new GraphQLDirective({
   name: 'requires',
-  description: '',
   locations: [DirectiveLocation.FIELD_DEFINITION],
   args: {
     fields: {
       type: GraphQLNonNull(GraphQLString),
-      description: '',
     },
   },
 });
 
 export const ProvidesDirective = new GraphQLDirective({
   name: 'provides',
-  description: '',
   locations: [DirectiveLocation.FIELD_DEFINITION],
   args: {
     fields: {
       type: GraphQLNonNull(GraphQLString),
-      description: '',
     },
   },
 });

federation-js/src/service/printFederatedSchema.ts

@@ -342,7 +342,7 @@ function printDescription(
     | GraphQLUnionType,
   indentation: string = '',
 ): string {
-  if (!def.description) {
+  if (def.description == null) {
     return '';
   }
 

federation-js/src/types.ts

@@ -76,7 +76,6 @@ export const entitiesField: GraphQLFieldConfig<any, any> = {
       type: new GraphQLNonNull(new GraphQLList(new GraphQLNonNull(AnyType))),
     },
   },
-  description: '',
   resolve(_source, { representations }, context, info) {
     return representations.map((reference: { __typename: string } & object) => {
       const { __typename } = reference;