[FIX] mail: fix display of unnecessary : in message about activity done

If the activity had no summary the messages has a : leading to nothing
which is not very nice. If there is no summary, only display the "done
by" message followed directly by the feedback.

addons/mail/views/mail_templates.xml

@@ -81,7 +81,7 @@
 <div>
     <p>
         <strong><span t-attf-class="fa #{activity.activity_type_id.icon}"/></strong> <span t-field="activity.activity_type_id.name"/> done
-        by <span t-field="activity.user_id.name"/>:
+        by <span t-field="activity.user_id.name"/><span t-if="activity.summary">:</span>
         <span t-if="activity.summary" t-field="activity.summary"/>
     </p>
     <div t-field="activity.note" style="margin-left: 8px;"/>