Fix stitchSchemas for when empty array of typeDefs are passed

If an empty array is passed, the GraphQL parser fails, because the
source is empty. Switch the typeDef check to check for length as well,
since an empty array is truthy.

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Fix visitEnumValue to allow modifying the enum value <br/>
   [@danielrearden](https://github.com/danielrearden) in [#1003](https://github.com/ardatan/graphql-tools/pull/1391)
 - Export `generateProxyingResolvers` from `@graphql-tools/wrap`.
+- Fix `stitchSchemas` from `@graphql-tools/stitch` from the case there the typeDefs array is empty.
 
 ### 5.0.0
 

packages/stitch/src/stitchSchemas.ts

@@ -51,7 +51,7 @@ export function stitchSchemas({
   }
 
   let schemaLikeObjects: Array<GraphQLSchema | SubschemaConfig | DocumentNode | GraphQLNamedType> = [...subschemas];
-  if (typeDefs) {
+  if (typeDefs && typeDefs.length) {
     schemaLikeObjects.push(buildDocumentFromTypeDefinitions(typeDefs, parseOptions));
   }
   if (types != null) {

packages/stitch/tests/stitchSchemas.test.ts

@@ -2982,6 +2982,36 @@ fragment BookingFragment on Booking {
     });
   });
 
+  describe('empty typeDefs array', () => {
+    test('works', async () => {
+      const typeDefs = `
+      type Query {
+        book: Book
+      }
+      type Book {
+        category: String!
+      }
+    `;
+      let schema = makeExecutableSchema({ typeDefs });
+
+      const resolvers = {
+        Query: {
+          book: () => ({ category: 'Test' }),
+        },
+      };
+
+      schema = stitchSchemas({
+        schemas: [schema],
+        resolvers,
+        typeDefs: [],
+      });
+
+      const result = await graphql(schema, '{ book { cat: category } }');
+
+      expect(result.data.book.cat).toBe('Test');
+    });
+  });
+
   describe('new root type name', () => {
     test('works', async () => {
       let bookSchema = makeExecutableSchema({