_is_saved fails when pk is 0.

[Balthasar-eu]

When the primary key is 0 the _is_saved method will always return false, even though 0 is a valid value for a primary key field.
I would recommend changing this line:

django-mptt/mptt/models.py

Line 803 in 32a5ecc

if not self.pk or self._mpttfield('tree_id') is None:

from if not self.pk or self._mpttfield('tree_id') is None: to if self.pk not None or self._mpttfield('tree_id') is None: or if (not self.pk and self.pk not 0) or self._mpttfield('tree_id') is None: