raft/raft_sys_table_storage: fix loading term/vote and snapshot from empty state
raft/raft_sys_table_storage: fix loading term/vote and snapshot from empty state

When a raft server is started for the first time and there isn't
any persisted state yet, provide default return values for
`load_term_and_vote` and `load_snapshot`. The code currently
does not handle this corner case correctly and fail with an
exception.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla