Add optional column on tree view

issue10989
review374321002

view/sale_line_tree.xml

@@ -3,7 +3,7 @@
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 <data>
     <xpath expr="//field[@name='unit_price']" position="after">
-        <field name="secondary_quantity" symbol="secondary_unit"/>
-        <field name="secondary_unit_price"/>
+        <field name="secondary_quantity" symbol="secondary_unit" optional="1"/>
+        <field name="secondary_unit_price" optional="1"/>
     </xpath>
 </data>