GraphQLEnumType: Drop support for 'undefined' custom value (#2148)

Motivation: #1383 (comment)
Fixes #1367

Author: IvanGoncharov

src/type/__tests__/definition-test.js

@@ -538,7 +538,8 @@ describe('Type System: Enums', () => {
       name: 'EnumWithNullishValue',
       values: {
         NULL: { value: null },
-        UNDEFINED: { value: undefined },
+        NAN: { value: NaN },
+        NO_CUSTOM_VALUE: { value: undefined },
       },
     });
 
@@ -553,9 +554,18 @@ describe('Type System: Enums', () => {
         astNode: undefined,
       },
       {
-        name: 'UNDEFINED',
+        name: 'NAN',
         description: undefined,
-        value: undefined,
+        value: NaN,
+        isDeprecated: false,
+        deprecationReason: undefined,
+        extensions: undefined,
+        astNode: undefined,
+      },
+      {
+        name: 'NO_CUSTOM_VALUE',
+        description: undefined,
+        value: 'NO_CUSTOM_VALUE',
         isDeprecated: false,
         deprecationReason: undefined,
         extensions: undefined,

src/type/definition.js

@@ -1290,7 +1290,7 @@ function defineEnumValues(
     return {
       name: valueName,
       description: valueConfig.description,
-      value: 'value' in valueConfig ? valueConfig.value : valueName,
+      value: valueConfig.value !== undefined ? valueConfig.value : valueName,
       isDeprecated: Boolean(valueConfig.deprecationReason),
       deprecationReason: valueConfig.deprecationReason,
       extensions: valueConfig.extensions && toObjMap(valueConfig.extensions),

src/utilities/__tests__/valueFromAST-test.js

@@ -64,8 +64,8 @@ describe('valueFromAST', () => {
       GREEN: { value: 2 },
       BLUE: { value: 3 },
       NULL: { value: null },
-      UNDEFINED: { value: undefined },
       NAN: { value: NaN },
+      NO_CUSTOM_VALUE: { value: undefined },
     },
   });
 
@@ -76,8 +76,8 @@ describe('valueFromAST', () => {
     testCase(testEnum, '"BLUE"', undefined);
     testCase(testEnum, 'null', null);
     testCase(testEnum, 'NULL', null);
-    testCase(testEnum, 'UNDEFINED', undefined);
     testCase(testEnum, 'NAN', NaN);
+    testCase(testEnum, 'NO_CUSTOM_VALUE', 'NO_CUSTOM_VALUE');
   });
 
   // Boolean!