Empty 'errors' list in introspection fix (#741)

* Fix for introspection when the client returns an empty errors list rather than null

* Update introspection.ts

Author: mhdubose

packages/apollo-language-server/src/schema/providers/introspection.ts

@@ -34,7 +34,7 @@ export class IntrospectionSchemaProvider implements GraphQLSchemaProvider {
       })
     )) as ExecutionResult<IntrospectionQuery>;
 
-    if (errors) {
+    if (errors && errors.length) {
       // XXX better error handling of GraphQL errors
       throw new Error(errors.map(({ message }: Error) => message).join("\n"));
     }