[MINOR][ML] Fixed MLlib build warnings

## What changes were proposed in this pull request?

Fixes to eliminate warnings during package and doc builds.

## How was this patch tested?

Existing unit tests

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12263 from jkbradley/warning-cleanups.

examples/src/main/java/org/apache/spark/examples/mllib/JavaStratifiedSamplingExample.java

@@ -35,6 +35,7 @@ public class JavaStratifiedSamplingExample {
     SparkConf conf = new SparkConf().setAppName("JavaStratifiedSamplingExample");
     JavaSparkContext jsc = new JavaSparkContext(conf);
 
+    @SuppressWarnings("unchecked")
     // $example on$
     List<Tuple2<Integer, Character>> list = new ArrayList<>(
       Arrays.<Tuple2<Integer, Character>>asList(

mllib/src/test/scala/org/apache/spark/ml/tree/impl/RandomForestSuite.scala

@@ -327,7 +327,9 @@ class RandomForestSuite extends SparkFunSuite with MLlibTestSparkContext {
       case n: InternalNode => n.split match {
         case s: CategoricalSplit =>
           assert(s.leftCategories === Array(1.0))
+        case _ => throw new AssertionError("model.rootNode.split was not a CategoricalSplit")
       }
+      case _ => throw new AssertionError("model.rootNode was not an InternalNode")
     }
   }
 
@@ -352,6 +354,7 @@ class RandomForestSuite extends SparkFunSuite with MLlibTestSparkContext {
         assert(n.leftChild.isInstanceOf[InternalNode])
         assert(n.rightChild.isInstanceOf[InternalNode])
         Array(n.leftChild.asInstanceOf[InternalNode], n.rightChild.asInstanceOf[InternalNode])
+      case _ => throw new AssertionError("rootNode was not an InternalNode")
     }
 
     // Single group second level tree construction.