Fix null value for tags/generic_attributes

git-svn-id: https://redmine.imt-atlantique.fr/svn/xaal/code/Python/branches/fork@1758 b32b6428-25c9-4566-ad07-03861ab6144f

apps/dashboard/xaal/dashboard/static/tags/generic_attrs.tag

@@ -3,7 +3,10 @@
 <div class="generic-attrs">
 <table width=100%>
   <tr><th>Name</th><th>value</th></tr>
-  <tr each="{ value, key in attributes }"><td>{ key }</td><td>{ value.toString() }</td></tr>
+  <tr each="{ value, key in attributes }">
+    <td>{ key }</td>
+    <td> { value == null ? 'null' : value.toString()} </td>
+  </tr>
 </table>
 </div>