[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.

(cherry picked from commit d24e7364)
Signed-off-by: Reynold Xin <rxin@databricks.com>