[FIX] website_sale: multi-image - enable first image in carousel

Before this commit, the first thumbnail from the multi image carousel was
always disabled since there are no atribute data-slide-to.

Now we force a '0' string instead of 0 falsy value to have explicitely
data-slide-to="0" after the qweb rendering.

addons/website_sale/views/templates.xml

@@ -440,7 +440,7 @@
                                     <li t-if="variant_img" data-target="#o-carousel-product" data-slide-to="0" class="active">
                                         <img class="img img-responsive js_variant_img_small" t-attf-src="/website/image/product.product/{{product.product_variant_id.id}}/image/90x90" t-att-alt="product.name"/>
                                     </li>
-                                    <li data-target="#o-carousel-product" t-att-data-slide-to="1 if variant_img else 0" t-att-class="'' if variant_img else 'active'">
+                                    <li data-target="#o-carousel-product" t-att-data-slide-to="1 if variant_img else '0'" t-att-class="'' if variant_img else 'active'">
                                         <img class="img img-responsive" t-attf-src="/website/image/product.template/{{product.id}}/image/90x90" t-att-alt="product.name"/>
                                     </li>
                                     <t t-if="len(image_ids)" t-foreach="image_ids" t-as="pimg">