Unable to add email subscriber to build notifications with existing subscriptions

### Environment information

```plain text
N/A
```

### Describe the bug

When attempting to add a new email subscriber to build notifications, the UI prevents saving due to validation errors with existing Lambda function ARN subscriptions.

The Lambda function subscription was manually added to the SNS topic created by Amplify.

### Reproduction steps

1. Create or start with an existing Amplify app, add email notifications to it
2. Review the SNS topic created in your AWS account
3. Add a Lambda function as a subscriber on the SNS topic
4. Amplify console lists all subscriptions
5. Add a new email subscriber from the Amplify console
6. Save

Start by reproducing the failure in the Amplify console with an SNS topic containing an existing Lambda function ARN subscription, then add an email subscriber and inspect the validation errors. Done means the console saves the new email subscription without rejecting the existing Lambda subscription.

Written by the indexing model from the issue text.