Skip nonsense screenshots when no theme is loaded

Change-Id: Ida91a9f75c6d36009c3b3e95b4117a2f1ba28502

uitest/src/com/vaadin/tests/themes/ThemeChangeOnTheFlyTest.java

@@ -71,7 +71,7 @@ public class ThemeChangeOnTheFlyTest extends MultiBrowserTest {
     public void reindeerToNullToReindeer() throws IOException {
         openTestURL();
 
-        changeThemeAndCompare("null");
+        changeTheme("null");
         changeThemeAndCompare("reindeer");
     }