fix(sync): error when rocks.toml has no rocks or plugins entries

[mrcjkb]

Fixes #291

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

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• For example, fix(installer): some installer bugfix

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If applicable:

• [x] Tested
  • [x] Tests have been added.
  • [x] Tested manually (steps in PR description).
• [ ] Updated documentation.