[FIX] mass_mailing_partner: Don't check if no category

Following 4de3795, as now `tag_ids` are not lost, the comparison is incorrect due to
partner not having category_id, and then keep it previous tags in mass mailing contact.
We add this check for avoiding problems.

mass_mailing_partner/tests/base.py

@@ -52,5 +52,6 @@ class BaseCase(TransactionCase):
                                  mailing_contact.company_name)
             self.assertEqual(mailing_contact.partner_id.country_id,
                              mailing_contact.country_id)
+            if mailing_contact.partner_id.category_id:
                 self.assertEqual(mailing_contact.partner_id.category_id,
                                  mailing_contact.tag_ids)