Fix check for is_sorted in treebeard/admin.py, the default value is an empty list and this was checking for None.
Fix check for is_sorted in treebeard/admin.py, the default value is an empty list and this was checking for None.

skirsdeda/django-treebeard,tabo/django-treebeard,tabo/django-treebeard,django-treebeard/django-treebeard,skirsdeda/django-treebeard,tabo/django-treebeard,tabo/django-treebeard,django-treebeard/django-treebeard,skirsdeda/django-treebeard,django-treebeard/django-treebeard,skirsdeda/django-treebeard