Don't check #present? of ActiveRecord objects because it's always true

(or the variable is assigned nil and it's always false)

While the rails maintainers argue about whether it's a good idea
rails/rails#35059

I found this because out of curiosity I ran our test suite with
ApplicationRecord#present? raising in our projects

Author: robotdana

Diff for: lib/closure_tree/hierarchy_maintenance.rb

@@ -23,7 +23,7 @@ def _ct_validate
       if !(defined? @_ct_skip_cycle_detection) &&
         !new_record? && # don't validate for cycles if we're a new record
         changes[_ct.parent_column_name] && # don't validate for cycles if we didn't change our parent
-        parent.present? && # don't validate if we're root
+        parent && # don't validate if we're root
         parent.self_and_ancestors.include?(self) # < this is expensive :\
         errors.add(_ct.parent_column_sym, I18n.t('closure_tree.loop_error', default: 'You cannot add an ancestor as a descendant'))
       end