Fix nil panic in integration test

[VinozzZ]

What does this change

Sometimes in integration test, if a mongo instance can't be successfully created, the Close() would return a nil panic.
This PR changes to only call Close when we are sure mongo is running

What issue does it fix

Notes for the reviewer

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• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR

[carolynvs]

Thanks!