[FIX] mass_mailing_partner: Don't match partner if no email

It doesn't make sense and even more, it crashed.

Fixes #644

mass_mailing_partner/__manifest__.py

@@ -6,7 +6,7 @@
 
 {
     "name": "Link partners with mass-mailing",
-    "version": "13.0.1.0.2",
+    "version": "13.0.1.0.3",
     "author": "Tecnativa, " "Odoo Community Association (OCA)",
     "website": "https://github.com/OCA/social",
     "license": "AGPL-3",

mass_mailing_partner/models/mailing_contact.py

@@ -100,6 +100,8 @@ class MailingContact(models.Model):
 
     def _set_partner(self):
         self.ensure_one()
+        if not self.email:
+            return
         m_partner = self.env["res.partner"]
         # Look for a partner with that email
         email = self.email.strip()