Add optional column on tree view

issue10989
review374321002

view/product_uom_tree.xml

@@ -3,6 +3,6 @@
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 <data>
     <xpath expr="//field[@name='symbol']" position="after">
-        <field name="unece_code"/>
+        <field name="unece_code" optional="1"/>
     </xpath>
 </data>