fix(fill): fix --collect-only after introducing FixtureOutput class

[danceratopz]

🗒️ Description

Follow-up bug fix to:

• refactor(fill): encapsulate fixture output options in FixtureOutput class #1471

Adds a CliRunner-based module test to catch the bug, which is fixed by creating the FixtureOutput object before exiting early if running with --collect-only.

🔗 Related Issues

Introduced in:

• refactor(fill): encapsulate fixture output options in FixtureOutput class #1471.

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.

[marioevz]

LGTM, thanks for the unit test :)