## Problem
In #9786 we stop storing SLRUs on non-zero shards.
However, there was one code path during ingest that still tries to
enumerate SLRU relations on all shards. This fails if it sees a tenant
who has never seen any write to an SLRU, or who has done such thorough
compaction+GC that it has dropped its SLRU directory key.
## Summary of changes
- Avoid trying to list SLRU relations on nonzero shards