mkpipeline: agent_query_rules can be set, but empty

[def-]

Same issue/fix as in https://github.com/MaterializeInc/i2/pull/2876

Seen in https://buildkite.com/materialize/test/builds/110421#0199c994-1a7f-4c2b-803e-babf0f5abf6c

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.