[SPARK-20615][ML][TEST] SparseVector.argmax throws IndexOutOfBoundsException

## What changes were proposed in this pull request?

Added a check for for the number of defined values.  Previously the argmax function assumed that at least one value was defined if the vector size was greater than zero.

## How was this patch tested?

Tests were added to the existing VectorsSuite to cover this case.

Author: Jon McLean <jon.mclean@atsid.com>

Closes #17877 from jonmclean/vectorArgmaxIndexBug.

mllib-local/src/main/scala/org/apache/spark/ml/linalg/Vectors.scala

@@ -657,6 +657,8 @@ class SparseVector @Since("2.0.0") (
   override def argmax: Int = {
     if (size == 0) {
       -1
+    } else if (numActives == 0) {
+      0
     } else {
       // Find the max active entry.
       var maxIdx = indices(0)

mllib-local/src/test/scala/org/apache/spark/ml/linalg/VectorsSuite.scala

@@ -125,6 +125,13 @@ class VectorsSuite extends SparkMLFunSuite {
 
     val vec8 = Vectors.sparse(5, Array(1, 2), Array(0.0, -1.0))
     assert(vec8.argmax === 0)
+
+    // Check for case when sparse vector is non-empty but the values are empty
+    val vec9 = Vectors.sparse(100, Array.empty[Int], Array.empty[Double]).asInstanceOf[SparseVector]
+    assert(vec9.argmax === 0)
+
+    val vec10 = Vectors.sparse(1, Array.empty[Int], Array.empty[Double]).asInstanceOf[SparseVector]
+    assert(vec10.argmax === 0)
   }
 
   test("vector equals") {

mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala

@@ -846,6 +846,8 @@ class SparseVector @Since("1.0.0") (
   override def argmax: Int = {
     if (size == 0) {
       -1
+    } else if (numActives == 0) {
+      0
     } else {
       // Find the max active entry.
       var maxIdx = indices(0)

mllib/src/test/scala/org/apache/spark/mllib/linalg/VectorsSuite.scala

@@ -122,6 +122,13 @@ class VectorsSuite extends SparkFunSuite with Logging {
 
     val vec8 = Vectors.sparse(5, Array(1, 2), Array(0.0, -1.0))
     assert(vec8.argmax === 0)
+
+    // Check for case when sparse vector is non-empty but the values are empty
+    val vec9 = Vectors.sparse(100, Array.empty[Int], Array.empty[Double]).asInstanceOf[SparseVector]
+    assert(vec9.argmax === 0)
+
+    val vec10 = Vectors.sparse(1, Array.empty[Int], Array.empty[Double]).asInstanceOf[SparseVector]
+    assert(vec10.argmax === 0)
   }
 
   test("vector equals") {