supporting reading file of size zero

weed/server/volume_server_handlers_read.go

@@ -67,7 +67,7 @@ func (vs *VolumeServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 	cookie := n.Cookie
 	count, e := vs.store.ReadVolumeNeedle(volumeId, n)
 	glog.V(4).Infoln("read bytes", count, "error", e)
-	if e != nil || count <= 0 {
+	if e != nil || count < 0 {
 		glog.V(0).Infoln("read error:", e, r.URL.Path)
 		w.WriteHeader(http.StatusNotFound)
 		return