Display variant name only when set

The previous method was falling back to the product name which is
displayed already.

app/assets/javascripts/templates/shop_variant.html.haml

@@ -1,6 +1,6 @@
 .variants.row
   .small-3.medium-4.large-6.columns.variant-name
-    .inline {{ ::variant.name_to_display }}
+    .inline {{ ::variant.display_name }}
     .variant-unit {{ ::variant.unit_to_display }}
     .bulk-buy.inline{"ng-if" => "::variant.product.group_buy"}
       %i.ofn-i_056-bulk><