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

## What changes were proposed in this pull request?

This is a follow-up PR of #15741 in order to keep `nextPowerOf2` consistent.

**Before**
```
nextPowerOf2(0) => 2
nextPowerOf2(1) => 1
nextPowerOf2(2) => 2
nextPowerOf2(3) => 4
nextPowerOf2(4) => 4
nextPowerOf2(5) => 8
```

**After**
```
nextPowerOf2(0) => 1
nextPowerOf2(1) => 1
nextPowerOf2(2) => 2
nextPowerOf2(3) => 4
nextPowerOf2(4) => 4
nextPowerOf2(5) => 8
```

## How was this patch tested?

N/A

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15754 from dongjoon-hyun/SPARK-18200-2.

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

@@ -272,7 +272,7 @@ class OpenHashSet[@specialized(Long, Int) T: ClassTag](
 
   private def nextPowerOf2(n: Int): Int = {
     if (n == 0) {
-      2
+      1
     } else {
       val highBit = Integer.highestOneBit(n)
       if (highBit == n) n else highBit << 1