lingo: don't display invoice subject if it has none (#13741)

combo/apps/lingo/templates/lingo/combo/item.html

@@ -8,11 +8,13 @@
     </h2>
     <form action="{% url 'lingo-pay' %}" method="POST">
       <div id="item">
+        {% if item.subject %}
         <h4 class="label">
           {% blocktrans with label=item.subject %}
           Label: {{ label }}
           {% endblocktrans %}
         </h4>
+        {% endif %}
         <div class="total_amount">
           {% blocktrans with amount=item.total_amount|floatformat:"2" %}
           Total amount: <span class="amount">{{ amount }}€</span>