feat: allow runAsync with null newMessage

Author: wenhaozhao

core/src/main/java/com/google/adk/runner/Runner.java

@@ -203,7 +203,8 @@ public Flowable<Event> runAsync(String userId, String sessionId, Content newMess
    * @param runConfig Configuration for the agent run.
    * @return A Flowable stream of {@link Event} objects generated by the agent during execution.
    */
-  public Flowable<Event> runAsync(Session session, Content newMessage, RunConfig runConfig) {
+  public Flowable<Event> runAsync(
+      Session session, @Nullable Content newMessage, RunConfig runConfig) {
     Span span = Telemetry.getTracer().spanBuilder("invocation").startSpan();
     try (Scope scope = span.makeCurrent()) {
       return Flowable.just(session)
@@ -213,7 +214,7 @@ public Flowable<Event> runAsync(Session session, Content newMessage, RunConfig r
                 InvocationContext invocationContext =
                     newInvocationContext(
                         sess,
-                        Optional.of(newMessage),
+                        Optional.ofNullable(newMessage),
                         /* liveRequestQueue= */ Optional.empty(),
                         runConfig);