Fixes#6219
superjson.parse returns undefined when given plain JSON (stored by older
versions before the superjson serializer was introduced). Without the null
guard, accessing .buster on undefined throws "Cannot read properties of
undefined (reading 'buster')", which is caught and logged as
"Failed to hydrate query cache" — contributing to instability on upgrade.
Also remove now-redundant optional chaining on persisted.clientState since
persisted is guaranteed non-null at that point.
ajnart
1fe07f8d79