polish(ast): prefer undefined over empty arrays (#4206)

see:
#2405 (comment)

Manually created ASTs always allowed undefined in place of empty arrays;
this change simply updates the parser to more closely follow the manual
approach. This is therefore not technically a breaking change, but
considering that there may be tools not aware of this, we have labelled
it a BREAKING_CHANGE to highlight it for consumers.

This closes #2203 as our tests now cover the undefined case by default
whenever there is an empty array.

Author: yaacovCR

src/execution/execute.ts

@@ -561,8 +561,6 @@ export function validateExecutionArgs(
     return [new GraphQLError('Must provide an operation.')];
   }
 
-  // FIXME: https://github.com/graphql/graphql-js/issues/2203
-  /* c8 ignore next */
   const variableDefinitions = operation.variableDefinitions ?? [];
   const hideSuggestions = args.hideSuggestions ?? false;
 

src/execution/values.ts

@@ -224,8 +224,6 @@ export function experimentalGetArgumentValues(
 ): { [argument: string]: unknown } {
   const coercedValues: { [argument: string]: unknown } = {};
 
-  // FIXME: https://github.com/graphql/graphql-js/issues/2203
-  /* c8 ignore next */
   const argumentNodes = node.arguments ?? [];
   const argNodeMap = new Map(argumentNodes.map((arg) => [arg.name.value, arg]));
 

src/language/__tests__/parser-test.ts

@@ -259,8 +259,8 @@ describe('Parser', () => {
           loc: { start: 0, end: 40 },
           operation: 'query',
           name: undefined,
-          variableDefinitions: [],
-          directives: [],
+          variableDefinitions: undefined,
+          directives: undefined,
           selectionSet: {
             kind: Kind.SELECTION_SET,
             loc: { start: 0, end: 40 },
@@ -290,7 +290,7 @@ describe('Parser', () => {
                     loc: { start: 9, end: 14 },
                   },
                 ],
-                directives: [],
+                directives: undefined,
                 selectionSet: {
                   kind: Kind.SELECTION_SET,
                   loc: { start: 16, end: 38 },
@@ -304,8 +304,8 @@ describe('Parser', () => {
                         loc: { start: 22, end: 24 },
                         value: 'id',
                       },
-                      arguments: [],
-                      directives: [],
+                      arguments: undefined,
+                      directives: undefined,
                       selectionSet: undefined,
                     },
                     {
@@ -317,8 +317,8 @@ describe('Parser', () => {
                         loc: { start: 30, end: 34 },
                         value: 'name',
                       },
-                      arguments: [],
-                      directives: [],
+                      arguments: undefined,
+                      directives: undefined,
                       selectionSet: undefined,
                     },
                   ],
@@ -349,8 +349,8 @@ describe('Parser', () => {
           loc: { start: 0, end: 29 },
           operation: 'query',
           name: undefined,
-          variableDefinitions: [],
-          directives: [],
+          variableDefinitions: undefined,
+          directives: undefined,
           selectionSet: {
             kind: Kind.SELECTION_SET,
             loc: { start: 6, end: 29 },
@@ -364,8 +364,8 @@ describe('Parser', () => {
                   loc: { start: 10, end: 14 },
                   value: 'node',
                 },
-                arguments: [],
-                directives: [],
+                arguments: undefined,
+                directives: undefined,
                 selectionSet: {
                   kind: Kind.SELECTION_SET,
                   loc: { start: 15, end: 27 },
@@ -379,8 +379,8 @@ describe('Parser', () => {
                         loc: { start: 21, end: 23 },
                         value: 'id',
                       },
-                      arguments: [],
-                      directives: [],
+                      arguments: undefined,
+                      directives: undefined,
                       selectionSet: undefined,
                     },
                   ],