fix: snykTestOutput defaults to undefined + types

[lili2311]

• [ ] Tests written and linted ℹ︎
• [ ] Documentation written ℹ︎
• [x] Commit history is tidy ℹ︎

What this does

Move away from using empty string as a value not set, use undefined instead to avoid issues checking if a value is present.

[aarlaud]

:tada: This PR is included in version 1.7.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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