django_rbac: always prevent empty uuid (#31083)

src/django_rbac/models.py

@@ -57,6 +57,8 @@ class AbstractBase(models.Model):
         if len(self.slug) > 256:
             self.slug = self.slug[:252] + \
                 hashlib.md5(self.slug).hexdigest()[:4]
+        if not self.uuid:
+            self.uuid = utils.get_hex_uuid()
         return super(AbstractBase, self).save(*args, **kwargs)
 
     def natural_key(self):