Add no-arg constructor for UnboundedReadFromBoundedSource

This allows Kryo to work with the type, currently required by
the Apex runner.

runners/core-java/src/main/java/org/apache/beam/runners/core/UnboundedReadFromBoundedSource.java

@@ -118,7 +118,12 @@ public void populateDisplayData(DisplayData.Builder builder) {
   public static class BoundedToUnboundedSourceAdapter<T>
       extends UnboundedSource<T, BoundedToUnboundedSourceAdapter.Checkpoint<T>> {
 
-    private BoundedSource<T> boundedSource;
+    @SuppressWarnings("unused") // for Kryo
+    private BoundedToUnboundedSourceAdapter() {
+      this.boundedSource = null;
+    }
+
+    private final BoundedSource<T> boundedSource;
 
     public BoundedToUnboundedSourceAdapter(BoundedSource<T> boundedSource) {
       this.boundedSource = boundedSource;