Fix unitialized optional result

Author: yunhanw-google

examples/java-matter-controller/java/src/com/matter/controller/commands/common/FutureResult.java

@@ -29,7 +29,7 @@
  * raised.
  */
 public class FutureResult {
-  private Optional<RealResult> realResult;
+  private Optional<RealResult> realResult = Optional.empty();
   private long timeoutMs = 0;
   private static Logger logger = Logger.getLogger(FutureResult.class.getName());