skip deltaBeat if dn is zero (#3630)

* skip deltaBeat
https://github.com/seaweedfs/seaweedfs/issues/3629

* fix GrpcPort

* skip url :0

* skip empty DataCenter or Rack

* skip empty heartbeat Ip

* dell msg add DataCenter

* comment todo

* fix

weed/pb/master_pb/master_helper.go

@@ -0,0 +1,5 @@
+package master_pb
+
+func (v *VolumeLocation) IsEmptyUrl() bool {
+	return v.Url == "" || v.Url == ":0"
+}