Sorry for the wait, I didn't have time until now to test the propsed solution.

With the answer of zbik pointing me in the direction, but the proposed solution not working, i had to modify it in two places to get it working:

<t t-set="is_discount" t-value="False"/>
<!-- This is important, otherwise is_discount will only be visible inside the foreach-loop and be undefined thereafter -->

<t t-foreach="o.order_line" t-as="l">
 <t t-if="l.discount"><t t-set="is_discount" t-value="True"/></t>
</t>

<!-- Table headers -->
<t t-if="is_discount">
  <th class="text-right">Discount</th>
</t> 

<!-- Actual table row -->
<t t-if="is_discount"> <!-- Instead of l.discount, otherwise there is a cell missing and following columns will be shifted -->
 <td class="text-right" groups="sale.group_discount_per_so_line"><t t-if="l.discount"><span t-field="l.discount"/>&#160;%</t></td>
</t>

PS: Sorry for posting this as answer, my karma doesn't allow me to comment yet :(