Don't allow adding to basket if the product has no price. (#4013)

src/oscar/templates/oscar/catalogue/partials/add_to_basket_form.html

@@ -5,7 +5,7 @@
 
 {% purchase_info_for_product request product as session %}
 
-{% if session.availability.is_available_to_buy %}
+{% if session.availability.is_available_to_buy and session.price.exists %}
     {% basket_form request product as basket_form %}
     <form id="add_to_basket_form" action="{% url 'basket:add' pk=product.pk  %}" method="post" class="add-to-basket mb-0">
         {% csrf_token %}

src/oscar/templates/oscar/catalogue/partials/add_to_basket_form_compact.html

@@ -4,7 +4,7 @@
 
 {% purchase_info_for_product request product as session %}
 
-{% if session.availability.is_available_to_buy %}
+{% if session.availability.is_available_to_buy and session.price.exists %}
     {% basket_form request product 'single' as basket_form %}
     <form action="{% url 'basket:add' pk=product.pk %}" method="post">
         {% csrf_token %}

src/oscar/templates/oscar/catalogue/partials/stock_record.html

@@ -15,6 +15,7 @@
 {% else %}
     <p class="price_color">&nbsp;</p>
 {% endif %}
+{% if session.price.exists %}
 <p class="{{ session.availability.code }} availability">
     {% if session.availability.is_available_to_buy %}
         <i class="fas fa-check-circle"></i>
@@ -27,3 +28,7 @@
         {{ session.availability.short_message }}
     {% endif %}
 </p>
+{% else %}
+    <i class="fas fa-ban"></i>
+    {% trans 'Unavailable' %}
+{% endif %}