ZODB Components: Get rid of WARNING message introduced in 448eea34.

WARNING product/ERP5Form/ListBox.py:987 DeprecationWarning: Each line of the
"Sortable Columns" field property must be in the form "<column_id> |
<cast_type>", where <cast_type> is one of ('', 'float').

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/BusinessTemplate_viewMigrateSourceCodeFromFilesystemDialog/listbox.xml

@@ -539,7 +539,12 @@
                 <item>
                     <key> <string>search_columns</string> </key>
                     <value>
-                      <list/>
+                      <list>
+                        <tuple>
+                          <string>None</string>
+                          <string></string>
+                        </tuple>
+                      </list>
                     </value>
                 </item>
                 <item>