avocado.core.tree: Modify is_leaf()

no need to compare size, simple `not self.children` does the same job.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Conflicts:
	avocado/core/tree.py

avocado/core/tree.py

@@ -101,7 +101,7 @@ class TreeNode(object):
 
     @property
     def is_leaf(self):
-        return len(self.children) == 0
+        return not self.children
 
     @property
     def root(self):