fix(route53-patterns): HttpsRedirect redirects to index.html

The CloudFront distribution should not specify a default root object when
used as a redirect from one domain to another.

Specify an empty string since the CDK default is `index.html`.

See aws#5700 (comment) and aws#5700 (comment)

packages/@aws-cdk/aws-route53-patterns/lib/website-redirect.ts

@@ -50,6 +50,7 @@ export class HttpsRedirect extends Construct {
       removalPolicy: RemovalPolicy.DESTROY,
     });
     const redirectDist = new CloudFrontWebDistribution(this, 'RedirectDistribution', {
+      defaultRootObject: '',
       originConfigs: [{
         behaviors: [{ isDefaultBehavior: true }],
         customOriginSource: {

packages/@aws-cdk/aws-route53-patterns/test/bucket-website-target.test.ts

@@ -30,8 +30,9 @@ test('create HTTPS redirect', () => {
   });
   expect(stack).toHaveResourceLike('AWS::CloudFront::Distribution', {
     DistributionConfig: {
-      Aliases: ['foo.example.com', 'baz.example.com']
-    }
+      Aliases: ['foo.example.com', 'baz.example.com'],
+      DefaultRootObject: '',
+    },
   });
   expect(stack).toHaveResource('AWS::Route53::RecordSet', {
     Name: 'foo.example.com.',