Make display_price optional on admin variants list

When there are variants with not fully configured prices (e.g. because
they are being created) the product variants page breaks without the
admin really knowing why.
This allows for the variants list to be displayed anyway.

backend/app/views/spree/admin/variants/_table.html.erb

@@ -27,7 +27,7 @@
         <% end %>
       </td>
       <td><%= variant.options_text %></td>
-      <td><%= variant.display_price.to_html %></td>
+      <td><%= variant.display_price&.to_html %></td>
       <td><%= variant.sku %></td>
       <td class="actions">
         <% if can?(:update, variant) %>

backend/spec/features/admin/products/variant_spec.rb

@@ -70,6 +70,18 @@
         expect(page).to have_content(discarded_variant.sku)
       end
     end
+
+    context 'displaying variants with unconfigured prices' do
+      let!(:variant) { create(:variant, sku: 'priceless_variant', product: product) }
+
+      before { variant.prices.delete_all }
+
+      it 'renders the listing correctly' do
+        visit spree.admin_product_variants_path(product)
+
+        expect(page).to have_content('priceless_variant')
+      end
+    end
   end
 
   context "editing existent variant" do