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

packages/apollo-federation/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: '',
     },
   },
 });

packages/apollo-federation/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;