Don't allocate an empty ArrayList for each Observable.empty call

Author: jbripley

src/main/java/rx/Observable.java

@@ -1057,7 +1057,7 @@ public final static <T> Observable<T> defer(Func0<Observable<T>> observableFacto
      * @see <a href="http://msdn.microsoft.com/en-us/library/hh229670.aspx">MSDN: Observable.Empty</a>
      */
     public final static <T> Observable<T> empty() {
-        return from(new ArrayList<T>());
+        return from(Collections.<T>emptyList());
     }
 
     /**