Update listener.rb

[amancevice]

What does this PR do?

Add :to_s to extraction of Function ARN in case this value is nil

Motivation

When invoking the function with SAM, context.invoked_function_arn can be nil which causes the existing code to error

Testing Guidelines

I tested this by manually creating a mock context object where context.invoked_function_arn == "" and saw that it worked

Additional Notes

Types of changes

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

Check all that apply

• [x] 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

[DylanLovesCoffee]

Thanks for the contribution @amancevice! It seems like the integration tests are failing for a reason unrelated to the PR, so once we get them to pass I'll merge this for you.