avocado.core.tree: Return self as root when no parents

self is a valid node and can be root of the tree.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -127,7 +127,7 @@ class TreeNode(object):
 
     def get_root(self):
         """ Get root of this tree """
-        root = None
+        root = self
         for root in self.iter_parents():
             pass
         return root