Add missing nullable annotation (#20505)

Author: jiahaog

shell/platform/android/io/flutter/plugin/common/MethodChannel.java

@@ -90,7 +90,7 @@ public void invokeMethod(@NonNull String method, @Nullable Object arguments) {
    * @param callback a {@link Result} callback for the invocation result, or null.
    */
   @UiThread
-  public void invokeMethod(String method, @Nullable Object arguments, Result callback) {
+  public void invokeMethod(String method, @Nullable Object arguments, @Nullable Result callback) {
     messenger.send(
         name,
         codec.encodeMethodCall(new MethodCall(method, arguments)),