chore: Make snapshot test fail when snapshot is empty

[lym953]

Motivation

The snapshot test doesn't fail when a generated snapshot is empty. As a result, empty snapshots can be added for new stacks as "correct" snapshots, making the test unable to capture unexpected changes.

For example, #309 fixed an empty snapshot.

What does this PR do?

Make the snapshot fail if a snapshot is empty.

Testing Guidelines

Steps:

1. In the stack lambda-function-stack-legacy-datadog-api.ts, change the stack name to something random, e.g.:

const stack = new ExampleStack(app, "lambda-function-stack-legacy-datadog-api-2", { env: env });

so lambda-function-stack-legacy-datadog-api.template.json won't be generated and will be initialized as an empty file by the script.
2. Run aws-vault exec sso-serverless-sandbox-account-admin -- scripts/run_integration_tests.sh

Result: The test failed.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog