[SPARK-18200][GRAPHX] Support zero as an initial capacity in OpenHashSet

## What changes were proposed in this pull request?

[SPARK-18200](https://issues.apache.org/jira/browse/SPARK-18200) reports Apache Spark 2.x raises `java.lang.IllegalArgumentException: requirement failed: Invalid initial capacity` while running `triangleCount`. The root cause is that `VertexSet`, a type alias of `OpenHashSet`, does not allow zero as a initial size. This PR loosens the restriction to allow zero.

## How was this patch tested?

Pass the Jenkins test with a new test case in `OpenHashSetSuite`.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15741 from dongjoon-hyun/SPARK-18200.

core/src/main/scala/org/apache/spark/util/collection/OpenHashSet.scala

@@ -48,7 +48,7 @@ class OpenHashSet[@specialized(Long, Int) T: ClassTag](
 
   require(initialCapacity <= OpenHashSet.MAX_CAPACITY,
     s"Can't make capacity bigger than ${OpenHashSet.MAX_CAPACITY} elements")
-  require(initialCapacity >= 1, "Invalid initial capacity")
+  require(initialCapacity >= 0, "Invalid initial capacity")
   require(loadFactor < 1.0, "Load factor must be less than 1.0")
   require(loadFactor > 0.0, "Load factor must be greater than 0.0")
 
@@ -271,8 +271,12 @@ class OpenHashSet[@specialized(Long, Int) T: ClassTag](
   private def hashcode(h: Int): Int = Hashing.murmur3_32().hashInt(h).asInt()
 
   private def nextPowerOf2(n: Int): Int = {
-    val highBit = Integer.highestOneBit(n)
-    if (highBit == n) n else highBit << 1
+    if (n == 0) {
+      2
+    } else {
+      val highBit = Integer.highestOneBit(n)
+      if (highBit == n) n else highBit << 1
+    }
   }
 }
 

core/src/test/scala/org/apache/spark/util/collection/OpenHashMapSuite.scala

@@ -49,9 +49,6 @@ class OpenHashMapSuite extends SparkFunSuite with Matchers {
     intercept[IllegalArgumentException] {
       new OpenHashMap[String, Int](-1)
     }
-    intercept[IllegalArgumentException] {
-      new OpenHashMap[String, String](0)
-    }
   }
 
   test("primitive value") {

core/src/test/scala/org/apache/spark/util/collection/OpenHashSetSuite.scala

@@ -176,4 +176,9 @@ class OpenHashSetSuite extends SparkFunSuite with Matchers {
     assert(set.size === 1000)
     assert(set.capacity > 1000)
   }
+
+  test("SPARK-18200 Support zero as an initial set size") {
+    val set = new OpenHashSet[Long](0)
+    assert(set.size === 0)
+  }
 }

core/src/test/scala/org/apache/spark/util/collection/PrimitiveKeyOpenHashMapSuite.scala

@@ -49,9 +49,6 @@ class PrimitiveKeyOpenHashMapSuite extends SparkFunSuite with Matchers {
     intercept[IllegalArgumentException] {
       new PrimitiveKeyOpenHashMap[Int, Int](-1)
     }
-    intercept[IllegalArgumentException] {
-      new PrimitiveKeyOpenHashMap[Int, Int](0)
-    }
   }
 
   test("basic operations") {