[FIX] rating: do not display False in email if no value

addons/rating_project/data/project_data.xml

@@ -87,7 +87,7 @@
                     </tr>
                 </table>
             </td></tr>
-            <tr><td style="padding:15px 20px 10px 20px;">${object.user_id.signature | safe}</td></tr>
+            <tr><td style="padding:15px 20px 10px 20px;">${object.user_id.signature or ''| safe}</td></tr>
         </tbody>
     </table>
     <table style="width:600px;margin:auto;text-align:center;font-size:12px;">