activities: Use isEmpty() to check whether the list of activities is empty.

Using Collection.size() to test for emptiness works, but using Collection.isEmpty() makes the code more readable and can be more performant

src/main/java/com/owncloud/android/ui/activities/ActivitiesActivity.java

@@ -252,7 +252,7 @@ public class ActivitiesActivity extends FileActivity implements ActivityListInte
         adapter.setActivityItems(activities, client, clear);
         this.nextPageUrl = nextPageUrl;
         // Hide the recyclerView if list is empty
-        if (activities.size() == 0) {
+        if (activities.isEmpty()) {
             recyclerView.setVisibility(View.INVISIBLE);
 
             emptyContentMessage.setText(noResultsMessage);