watcher send changes only on non nil

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

consul.go

@@ -247,12 +247,13 @@ func (w *consulWatcher) run() {
 					delete(dstmp, sk)
 				}
 			}
-
+			if len(dstmp) > 0 {
 				w.vchan <- dstmp
 				src = dst
 			}
 		}
 	}
+}
 
 func (w *consulWatcher) Next() (map[string]interface{}, error) {
 	select {