[FIX] website_blog: optimize crawler prevention

Commit d39856db prevents crawler to crawl and index blog tag URL which could
lead to possibly millions of URLs combinaison.

Still, it would crawl (but not index) second level of tags combinaison.

This commit ensure that crawling is prevented when already one tag on the URL.

addons/website_blog/views/website_blog_templates.xml

@@ -103,7 +103,7 @@
     <t t-call="website_blog.index">
         <t t-set="head">
             <link t-att-href="'/blog/%s/feed' % (blog.id)" type="application/atom+xml" rel="alternate" title="Atom Feed"/>
-            <meta t-if="len(active_tag_ids) > 1" name="robots" content="noindex, nofollow"/>
+            <meta t-if="len(active_tag_ids) > 0" name="robots" t-attf-content="#{len(active_tag_ids) > 1 and 'noindex,'} nofollow"/>
         </t>
         <div class="container">
             <t t-call="website.pager" >