a8n: Fix listing all ChangesetEvents resulting in empty results

Created by: mrnugget

This fixes #6011 (closed).

The panic in the bug report was caused by len(cs) being 0 and trying to access cs[:-1], leading to an index-out-of-range panic.

This happens only when Limit has been set to -1 before (it's incremented to 0 in the function that constructs the query) to list all changeset events and the query resulting in an empty result set.