treat it as a single node cluster if empty raft server name

possible fix for https://github.com/chrislusf/seaweedfs/issues/1118

weed/topology/topology.go

@@ -60,7 +60,12 @@ func NewTopology(id string, seq sequence.Sequencer, volumeSizeLimit uint64, puls
 
 func (t *Topology) IsLeader() bool {
 	if t.RaftServer != nil {
-		return t.RaftServer.State() == raft.Leader
+		if t.RaftServer.State() == raft.Leader {
+			return true
+		}
+		if t.RaftServer.Leader() == "" {
+			return true
+		}
 	}
 	return false
 }
@@ -75,7 +80,7 @@ func (t *Topology) Leader() (string, error) {
 
 	if l == "" {
 		// We are a single node cluster, we are the leader
-		return t.RaftServer.Name(), errors.New("Raft Server not initialized!")
+		return t.RaftServer.Name(), nil
 	}
 
 	return l, nil
@@ -152,7 +157,7 @@ func (t *Topology) ListCollections(includeNormalVolumes, includeEcVolumes bool)
 		t.ecShardMapLock.RUnlock()
 	}
 
-	for k, _ := range mapOfCollections {
+	for k := range mapOfCollections {
 		ret = append(ret, k)
 	}
 	return ret