Add optional column on tree view

issue10989
review374321002

view/company_list.xml

@@ -3,4 +3,6 @@
 this repository contains the full copyright notices and license terms. -->
 <tree>
     <field name="party" expand="1"/>
+    <field name="currency" optional="1"/>
+    <field name="timezone" optional="1"/>
 </tree>

view/employee_tree.xml

@@ -3,8 +3,8 @@
 this repository contains the full copyright notices and license terms. -->
 <tree>
     <field name="party" expand="1"/>
-    <field name="company" expand="1"/>
-    <field name="supervisor" expand="1"/>
-    <field name="start_date"/>
-    <field name="end_date"/>
+    <field name="company" expand="1" optional="1"/>
+    <field name="supervisor" expand="1" optional="1"/>
+    <field name="start_date" optional="1"/>
+    <field name="end_date" optional="1"/>
 </tree>

view/sequence_tree.xml

@@ -3,6 +3,6 @@
 this repository contains the full copyright notices and license terms. -->
 <data>
     <xpath expr="//field[@name='sequence_type']" position="after">
-        <field name="company" expand="1"/>
+        <field name="company" expand="1" optional="1"/>
     </xpath>
 </data>