fix(autoscaling): osType is wrong when using CloudformationInit with …

…launchTemplate (#20759)

Fix #20304

Like #20454 but add a test

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

packages/@aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts

@@ -1206,7 +1206,7 @@ export class AutoScalingGroup extends AutoScalingGroupBase implements
       this._role = this.launchTemplate?.role;
       this.grantPrincipal = this._role || new iam.UnknownPrincipal({ resource: this });
 
-      this.osType = this.launchTemplate?.osType || ec2.OperatingSystemType.UNKNOWN;
+      this.osType = this.launchTemplate?.osType ?? ec2.OperatingSystemType.UNKNOWN;
     } else {
       if (!props.machineImage) {
         throw new Error('Setting \'machineImage\' is required when \'launchTemplate\' and \'mixedInstancesPolicy\' is not set');

packages/@aws-cdk/aws-autoscaling/test/auto-scaling-group.test.ts

@@ -1732,6 +1732,32 @@ describe('auto scaling group', () => {
       asg.addSecurityGroup(mockSecurityGroup(stack));
     }).toThrow('You cannot add security groups when the Auto Scaling Group is created from a Launch Template.');
   });
+
+  test('Should not throw when LaunchTemplate is used with CloudformationInit', () => {
+    const stack = new cdk.Stack();
+
+    // WHEN
+    const lt = new LaunchTemplate(stack, 'LaunchTemplate', {
+      machineImage: new AmazonLinuxImage(),
+      instanceType: new ec2.InstanceType('t3.micro'),
+      userData: ec2.UserData.forLinux(),
+      securityGroup: ec2.SecurityGroup.fromSecurityGroupId(stack, 'ImportedSg', 'securityGroupId'),
+      role: iam.Role.fromRoleArn(stack, 'ImportedRole', 'arn:aws:iam::123456789012:role/MockRole'),
+    });
+
+    const cfInit = ec2.CloudFormationInit.fromElements(
+      ec2.InitCommand.shellCommand('/bash'),
+    );
+
+    // THEN
+    expect(() => new autoscaling.AutoScalingGroup(stack, 'Asg', {
+      launchTemplate: lt,
+      init: cfInit,
+      vpc: mockVpc(stack),
+      signals: autoscaling.Signals.waitForAll(),
+    })).not.toThrow();
+
+  });
 });
 
 function mockVpc(stack: cdk.Stack) {