fix(Item Group): allow root deletion

It was not possible to delete an empty, unused Item Group without any
children, if it was one of possibly multiple roots of the Item Group tree.
This fix allows deleting a root Item Group.

(cherry picked from commit fd2c272)

erpnext/setup/doctype/item_group/item_group.py

@@ -73,7 +73,7 @@ def make_route(self):
 			return self.route
 
 	def on_trash(self):
-		NestedSet.on_trash(self)
+		NestedSet.on_trash(self, allow_root_deletion=True)
 		WebsiteGenerator.on_trash(self)
 		self.delete_child_item_groups_key()