do not send variables with value undefined

graphql-js converts these to null
See graphql/graphql-js#2533

src/delegate/createRequest.ts

@@ -113,10 +113,13 @@ export function createRequest({
         sourceSchema,
         def.type as NamedTypeNode,
       ) as GraphQLInputType;
-      newVariables[varName] = serializeInputValue(
+      const serializedValue = serializeInputValue(
         varType,
         variableValues[varName],
       );
+      if (serializedValue !== undefined) {
+        newVariables[varName] = serializedValue;
+      }
     });
   }
 

src/delegate/transforms/FilterToSchema.ts

@@ -142,7 +142,10 @@ function filterToSchema(
   });
 
   const newVariables = usedVariables.reduce((acc, variableName) => {
-    acc[variableName] = variables[variableName];
+    const variableValue = variables[variableName];
+    if (variableValue !== undefined) {
+      acc[variableName] = variableValue;
+    }
     return acc;
   }, {});
 

src/stitch/typeFromAST.ts

@@ -188,7 +188,7 @@ function makeValues(
       [node.name.value]: {
         type: createStub(node.type, 'input'),
         defaultValue:
-          node.defaultValue != null
+          node.defaultValue !== undefined
             ? valueFromASTUntyped(node.defaultValue)
             : undefined,
         description: getDescription(node, backcompatOptions),

src/test/alternateStitchSchemas.test.ts

@@ -426,6 +426,72 @@ describe('transform object fields', () => {
   });
 });
 
+describe('optional arguments', () => {
+  const schema = makeExecutableSchema({
+    typeDefs: `
+      enum Arg {
+        possibleArg
+      }
+      type Query {
+        test(arg: Arg): Boolean
+      }
+    `,
+    resolvers: {
+      Query: {
+        test: (_root, args, _context) => args.arg === undefined
+      }
+    }
+  });
+
+  const stitchedSchema = stitchSchemas({
+    schemas: [schema],
+  });
+
+  it('work with schema stitching', async () => {
+    const query = `
+      {
+        test
+      }
+    `;
+
+    const originalResult = await graphql(schema, query);
+    expect(originalResult.data.test).toEqual(true);
+
+    const stitchedResult = await graphql(stitchedSchema, query);
+    expect(stitchedResult.data.test).toEqual(true);
+  });
+
+  it('work with schema stitching when using variables', async () => {
+    const query = `
+      query test($arg: Arg) {
+        test(arg: $arg)
+      }
+    `;
+
+    const originalResult = await graphql(schema, query);
+    expect(originalResult.data.test).toEqual(true);
+
+    const stitchedResult = await graphql(stitchedSchema, query);
+    expect(stitchedResult.data.test).toEqual(true);
+  });
+
+  // See https://github.com/graphql/graphql-js/issues/2533
+  it('may not work as expected when explicitly passing in an undefined value', async () => {
+    const query = `
+      query test($arg: Arg) {
+        test(arg: $arg)
+      }
+    `;
+
+    const originalResult = await graphql(schema, query, {}, {}, { arg: undefined });
+    expect(originalResult.data.test).toEqual(false);
+
+    const stitchedResult = await graphql(stitchedSchema, query, {}, {}, { arg: undefined });
+    expect(stitchedResult.data.test).toEqual(false);
+  });
+});
+
+
 describe('default values', () => {
   test('should work to add a default value even when renaming root fields', async () => {
     const transformedPropertySchema = transformSchema(propertySchema, [

src/utils/updateArgument.ts

@@ -47,5 +47,9 @@ export function updateArgument(
     type: astFromType(argType),
   };
 
-  variableValues[varName] = newArg;
+  if (newArg === undefined) {
+    delete variableValues[varName];
+  } else {
+    variableValues[varName] = newArg;
+  }
 }