fix: don't pass callback as metadata when omitted

[nathanfranke]

Changes

If callback is passed but not metadata, reset metadata.

Verification

I changed this code manually on my generated files and it works exactly as expected.

[improbable-prow-robot]

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nathanfranke]

Still desired