Return empty array if there are no objects to list

[MichaelEischer]

What is the purpose of this change? What does it change?

Previously "null" was returned, which does not match the REST backend specification.

Was the change discussed in an issue or in the forum before?

Fixes #238

Checklist

• [x] I have enabled maintainer edits for this PR
• [x] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [x] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[MichaelEischer]

LGTM.