change(graph): changed null value graph option to default to null, closes #6868

public/app/plugins/panel/graph/module.ts

@@ -88,7 +88,7 @@ class GraphCtrl extends MetricsPanelCtrl {
       avg: false
     },
     // how null points should be handled
-    nullPointMode : 'connected',
+    nullPointMode : 'null',
     // staircase line mode
     steppedLine: false,
     // tooltip options