fix(cli): fix stack monitoring when the stack events do not have phsical resource id set (#27692)

`PhysicalResourceId` for a `StackEvent` is [not required](https://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_StackEvent.html) and can be empty. This causes error in stack deployment activty monitoring where we try to monitor nested stacks but empty `PhysicalResourceId` in the `StackEvent` causes it to fail with the error

```
[ValidationError]: 2 validation errors detected: 
Value '' at 'stackName' failed to satisfy constraint: Member must have length greater than or equal to 1; 
Value '' at 'stackName' failed to satisfy constraint: Member must satisfy regular expression pattern: [a-zA-Z][-a-zA-Z0-9]*|arn:[-a-zA-Z0-9:/._+]*
```
This PR adds a check to avoid that.

----

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

Author: Amplifiyer

packages/aws-cdk/lib/api/util/cloudformation/stack-activity-monitor.ts

@@ -252,8 +252,8 @@ export class StackActivityMonitor {
           });
 
           if (event.ResourceType === 'AWS::CloudFormation::Stack' && !CFN_SUCCESS_STATUS.includes(event.ResourceStatus ?? '')) {
-            // If the event is not for `this` stack, recursively call for events in the nested stack
-            if (event.PhysicalResourceId !== stackToPollForEvents) {
+            // If the event is not for `this` stack and has a physical resource Id, recursively call for events in the nested stack
+            if (event.PhysicalResourceId && event.PhysicalResourceId !== stackToPollForEvents) {
               await this.readNewEvents(event.PhysicalResourceId);
             }
           }

packages/aws-cdk/test/util/stack-monitor.test.ts

@@ -153,6 +153,43 @@ describe('stack monitor, collecting errors from events', () => {
     expect(monitor.errors).toStrictEqual(['actual failure error message', 'nested stack failed']);
   });
 
+  test('does not consider events without physical resource id for monitoring nested stacks', async () => {
+    const monitor = await testMonitorWithEventCalls([
+      (request) => {
+        expect(request.StackName).toStrictEqual('StackName');
+        return {
+          StackEvents: [
+            addErrorToStackEvent(
+              event(100), {
+                logicalResourceId: 'nestedStackLogicalResourceId',
+                physicalResourceId: '',
+                resourceType: 'AWS::CloudFormation::Stack',
+                resourceStatusReason: 'nested stack failed',
+              },
+            ),
+          ],
+        };
+      },
+      (request) => {
+        // Note that the second call happened for the top level stack instead of a nested stack
+        expect(request.StackName).toStrictEqual('StackName');
+        return {
+          StackEvents: [
+            addErrorToStackEvent(
+              event(101), {
+                logicalResourceId: 'OtherResource',
+                resourceType: 'Some::Other::Resource',
+                resourceStatusReason: 'some failure',
+              },
+            ),
+          ],
+        };
+      },
+    ]);
+
+    expect(monitor.errors).toStrictEqual(['nested stack failed', 'some failure']);
+  });
+
   test('does not check for nested stacks that have already completed successfully', async () => {
     const monitor = await testMonitorWithEventCalls([
       (request) => {