Compare element to replace new view tree from inherit application against None

lxml raises a warning about truth-testing on element.

Close #14429

trytond/trytond/ir/ui/view.py

@@ -347,7 +347,7 @@ class View(
                 position = element.get('position', 'inside')
                 new_tree = getattr(cls, '_inherit_apply_%s' % position)(
                     tree, element, target)
-                if new_tree:
+                if new_tree is not None:
                     tree = new_tree
         return tree