Make deprecated.reason non-nullable (#4299)

See graphql/graphql-spec#1040

Author: martinbonnin

Diff for: src/type/__tests__/introspection-test.ts

@@ -978,9 +978,13 @@ describe('Introspection', () => {
                   defaultValue: '"No longer supported"',
                   name: 'reason',
                   type: {
-                    kind: 'SCALAR',
-                    name: 'String',
-                    ofType: null,
+                    kind: 'NON_NULL',
+                    name: null,
+                    ofType: {
+                      kind: 'SCALAR',
+                      name: 'String',
+                      ofType: null,
+                    },
                   },
                 },
               ],

Diff for: src/type/directives.ts

@@ -241,7 +241,7 @@ export const GraphQLDeprecatedDirective: GraphQLDirective =
     ],
     args: {
       reason: {
-        type: GraphQLString,
+        type: new GraphQLNonNull(GraphQLString),
         description:
           'Explains why this element was deprecated, usually also including a suggestion for how to access supported similar data. Formatted using the Markdown syntax, as specified by [CommonMark](https://commonmark.org/).',
         defaultValue: DEFAULT_DEPRECATION_REASON,

Diff for: src/utilities/__tests__/printSchema-test.ts

@@ -803,7 +803,7 @@ describe('Type System Printer', () => {
         """
         Explains why this element was deprecated, usually also including a suggestion for how to access supported similar data. Formatted using the Markdown syntax, as specified by [CommonMark](https://commonmark.org/).
         """
-        reason: String = "No longer supported"
+        reason: String! = "No longer supported"
       ) on FIELD_DEFINITION | ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION | ENUM_VALUE
 
       """Exposes a URL that specifies the behavior of this scalar."""