s4-drepl: We won't need a working schema for empty replicas sent.

s4-drepl: We won't need a working schema for empty replicas sent.

Without this check, receiving empty replica leads to a situation
where we left with a working_schema attached to the ldb.

The problem here is that working_schema is not fully functional
schema cache and keeping it attached to the ldb may lead
to modules failing to accomplish their jobs