Add optional column on tree view

issue10989
review374321002

view/budget_list.xml

@@ -2,9 +2,9 @@
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
 <tree>
+    <field name="company" expand="1" optional="1"/>
     <field name="name" expand="2"/>
-    <field name="company" expand="1"/>
-    <field name="start_date"/>
-    <field name="end_date"/>
+    <field name="start_date" optional="0"/>
+    <field name="end_date" optional="0"/>
     <button name="copy_button"/>
 </tree>