[SPARK-22811][PYSPARK][ML] Fix pyspark.ml.tests failure when Hive is not available.

## What changes were proposed in this pull request?

pyspark.ml.tests is missing a py4j import. I've added the import and fixed the test that uses it. This test was only failing when testing without Hive.

## How was this patch tested?

Existing tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Bago Amirbekian <[email protected]>

Closes apache#19997 from MrBago/fix-ImageReaderTest2.

Author: MrBago

python/pyspark/ml/tests.py

@@ -44,6 +44,7 @@
 import numpy as np
 from numpy import abs, all, arange, array, array_equal, inf, ones, tile, zeros
 import inspect
+import py4j
 
 from pyspark import keyword_only, SparkContext
 from pyspark.ml import Estimator, Model, Pipeline, PipelineModel, Transformer, UnaryTransformer
@@ -1859,8 +1860,9 @@ class ImageReaderTest2(PySparkTestCase):
 
     @classmethod
     def setUpClass(cls):
-        PySparkTestCase.setUpClass()
+        super(ImageReaderTest2, cls).setUpClass()
         # Note that here we enable Hive's support.
+        cls.spark = None
         try:
             cls.sc._jvm.org.apache.hadoop.hive.conf.HiveConf()
         except py4j.protocol.Py4JError:
@@ -1873,8 +1875,10 @@ def setUpClass(cls):
 
     @classmethod
     def tearDownClass(cls):
-        PySparkTestCase.tearDownClass()
-        cls.spark.sparkSession.stop()
+        super(ImageReaderTest2, cls).tearDownClass()
+        if cls.spark is not None:
+            cls.spark.sparkSession.stop()
+            cls.spark = None
 
     def test_read_images_multiple_times(self):
         # This test case is to check if `ImageSchema.readImages` tries to