chore(CLI): nested stacks should be skipped by changeset diff (#28784)

#28741 accidentally broke the logic that skips them

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: comcalvi

packages/aws-cdk/lib/api/util/cloudformation.ts

@@ -317,7 +317,7 @@ export async function createDiffChangeSet(options: PrepareChangeSetOptions): Pro
       // eslint-disable-next-line no-console
       debug('This stack contains one or more nested stacks, falling back to template-only diff...');
 
-      //return undefined;
+      return undefined;
     }
   }
 

packages/aws-cdk/test/diff.test.ts

@@ -350,7 +350,6 @@ Resources
     const plainTextOutput = buffer.data.replace(/\x1B\[[0-?]*[ -/]*[@-~]/g, '')
       .replace(/[ \t]+$/mg, '');
     expect(plainTextOutput.trim()).toEqual(`Stack Parent
-Could not create a change set, will base the diff on template differences (run again with -v to see the reason)
 Resources
 [~] AWS::CloudFormation::Stack AdditionChild
  └─ [~] Resources