ui:fragment: Use isEmpty() to check if 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/fragment/FileDetailActivitiesFragment.java

@@ -243,7 +243,7 @@ public class FileDetailActivitiesFragment extends Fragment implements ActivityLi
 
                     activity.runOnUiThread(() -> {
                         populateList(activities, ownCloudClient, pageUrl == null);
-                        if (activities.size() > 0) {
+                        if (!activities.isEmpty()) {
                             empty.setVisibility(View.GONE);
                             list.setVisibility(View.VISIBLE);
                         } else {