[FIX] website_event_track: suggest recipient with an email

If an event track had a speaker assigned who had an email, and the
email field in the track itself was empty, Odoo was suggesting the
user to send an email to an empty recipient, making an error

Now it only suggests the email if there's one.

Fixes odoo/odoo#35941

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/website_event_track/models/event_track.py

@@ -155,7 +155,7 @@ class Track(models.Model):
     def message_get_suggested_recipients(self):
         recipients = super(Track, self).message_get_suggested_recipients()
         for track in self:
-            if track.partner_email != track.partner_id.email:
+            if track.partner_email and track.partner_email != track.partner_id.email:
                 track._message_add_suggested_recipient(recipients, email=track.partner_email, reason=_('Speaker Email'))
         return recipients