Currently, `peers_table_read_fixup` removes rows with no `host_id`, but
not with null `host_id`. Null host IDs are known to appear in system
tables, for example in `system.cluster_status` after a failed bootstrap.
We better make sure we handle them properly if they ever appear in
`system.peers`.
This commit guarantees that null UUID cannot belong to
`loaded_endpoints` in `storage_service::join_cluster`, which in
particular ensures that we throw a runtime error when a user sets
`recovery_leader` to null UUID during the recovery procedure. This is
handled by the code verifying that `recovery_leader` belongs to
`loaded_endpoints`.
(cherry picked from commit 23f59483b6)
1738f244d2