[SPARK-14219][GRAPHX] Fix `pickRandomVertex` not to fall into infinite loops...

[SPARK-14219][GRAPHX] Fix `pickRandomVertex` not to fall into infinite loops for graphs with one vertex

## What changes were proposed in this pull request?

Currently, `GraphOps.pickRandomVertex()` falls into infinite loops for graphs having only one vertex. This PR fixes it by modifying the following termination-checking condition.
```scala
-      if (selectedVertices.count > 1) {
+      if (selectedVertices.count > 0) {
```

## How was this patch tested?

Pass the Jenkins tests (including new test case).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12018 from dongjoon-hyun/SPARK-14219.

graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala

@@ -276,7 +276,7 @@ class GraphOps[VD: ClassTag, ED: ClassTag](graph: Graph[VD, ED]) extends Seriali
         if (Random.nextDouble() < probability) { Some(vidVvals._1) }
         else { None }
       }
-      if (selectedVertices.count > 1) {
+      if (selectedVertices.count > 0) {
         found = true
         val collectedVertices = selectedVertices.collect()
         retVal = collectedVertices(Random.nextInt(collectedVertices.length))

graphx/src/test/scala/org/apache/spark/graphx/GraphSuite.scala

@@ -404,4 +404,13 @@ class GraphSuite extends SparkFunSuite with LocalSparkContext {
       assert(sc.getPersistentRDDs.isEmpty)
     }
   }
+
+  test("SPARK-14219: pickRandomVertex") {
+    withSpark { sc =>
+      val vert = sc.parallelize(List((1L, "a")), 1)
+      val edges = sc.parallelize(List(Edge[Long](1L, 1L)), 1)
+      val g0 = Graph(vert, edges)
+      assert(g0.pickRandomVertex() === 1L)
+    }
+  }
 }