Fix dir=auto for convos with blank timestamps

Convos with no last_message_timestamp were being rendered/parsed as
having `data-timestamp="dir='auto'"`.

// FREEBIE

background.html

@@ -195,7 +195,7 @@
   <script type='text/x-tmpl-mustache' id='conversation-preview'>
       {{> avatar }}
       <div class='contact-details'>
-          <span class='last-timestamp' data-timestamp={{ last_message_timestamp }} dir='auto'> </span>
+          <span class='last-timestamp' data-timestamp='{{ last_message_timestamp }}' dir='auto' > </span>
           {{> contact_name_and_number }}
           {{ #unreadCount }}
             <span class='unread-count'>{{ unreadCount }}</span>