[FIX] digest, mass_mailing: fix crash about impossible 'unsubscribe'

Mailing statistics emails are build on digest layout. This layout contains a
reference to the digest unsubscribe mechanism, which obviously does not work
with mailing statistics emails as they currently have no opt-out mechanism.
This commit fixes that part of the template so that unsubscribe is displayed
only with digests.

Task-2582128 (Digest onboarding and usage improvement)
Task-2686586 (Repair mailing statistics email)

X-original-commit: febe4506
Part-of: odoo/odoo#79877

addons/digest/data/digest_data.xml

@@ -399,8 +399,14 @@
                 <t t-out="preference"/>
             </div>
             <div class="by_odoo">
-                Sent by <a href="https://www.odoo.com" target="_blank" class="odoo_link"><span class="odoo_link_text">Odoo</span></a> –
-                <a t-att-href="'/web#view_type=form&amp;model=digest.digest&amp;id=%s' % object.id" target="_blank" style="text-decoration: none;"><span style="color: #8f8f8f;">Unsubscribe</span></a>
+                Sent by <a href="https://www.odoo.com" target="_blank" class="odoo_link"><span class="odoo_link_text">Odoo</span></a>
+                <t t-if="object and object._name == 'digest.digest'">
+                    –
+                    <a t-att-href="'/web#view_type=form&amp;model=digest.digest&amp;id=%s' % object.id"
+                       target="_blank" style="text-decoration: none;">
+                        <span style="color: #8f8f8f;">Unsubscribe</span>
+                    </a>
+                </t>
             </div>
         </div>
     </div>