Add optional column on tree view

issue10989
review374321002

view/carrier_tree.xml

@@ -3,6 +3,6 @@
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 <data>
     <xpath expr="/tree/field[@name='party']" position="after">
-        <field name="shipping_service" expand="1"/>
+        <field name="shipping_service" expand="1" optional="1"/>
     </xpath>
 </data>