graph.mapVertices() function obtain edge triplets with null attribute

[James <al...@gmail.com>]

My code

```
// Initial the graph, assign a counter to each vertex that contains the
vertex id only
var anfGraph = graph.mapVertices { case (vid, _) =>
  val counter = new HyperLogLog(5)
  counter.offer(vid)
  counter
}

val nullVertex = anfGraph.triplets.filter(edge => edge.srcAttr ==
null).first
// There is an edge whose src attr is null

anfGraph.vertices.filter(_._1 == nullVertex).first
// I could see that the vertex has a not null attribute

// messages = anfGraph.aggregateMessages(msgFun, mergeMessage)   // <-
NullPointerException

```

My spark version:1.2.0

Alcaid