FIX: If a site has no logo, don't show a broken image in the digest.

app/helpers/user_notifications_helper.rb

@@ -28,6 +28,8 @@ module UserNotificationsHelper
   def logo_url
     logo_url = SiteSetting.digest_logo_url
     logo_url = SiteSetting.logo_url if logo_url.blank?
+
+    return nil if logo_url.blank?
     if logo_url !~ /http(s)?\:\/\//
       logo_url = "#{Discourse.base_url}#{logo_url}"
     end

app/views/user_notifications/digest.html.erb

@@ -2,7 +2,11 @@
 <tr>
   <td style="padding: 10px 10px; background-color: #<%= @header_color %>;">
     <a href="<%= Discourse.base_url %>">
-    <img src="<%= logo_url %>" style="max-height: 35px; min-height: 35px; height: 35px;" class='site-logo'></a>
+    <%- if logo_url.blank? %>
+      <%= SiteSetting.title %>
+    <%- else %>
+      <img src="<%= logo_url %>" style="max-height: 35px; min-height: 35px; height: 35px;" class='site-logo'></a>
+    <%- end %>
   </td>
 </tr>
 <tr>