Fix UI_driver.has_class exception. Handle situation when element has no class attribute

Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>

ipatests/test_webui/ui_driver.py

@@ -1908,7 +1908,8 @@ class UI_driver(object):
         """
         Check if el has CSS class
         """
-        return cls in el.get_attribute("class").split()
+        class_attr = el.get_attribute("class")
+        return bool(class_attr) and cls in class_attr.split()
 
     def skip(self, reason):
         """