[FIX] website_blog: feed when no posts

When no post exist for a blog, accessing `/blog/<id>/feed` crashes
because `blog.blog` has no field `post_date`.

opw-2066587

closes odoo/odoo#36591

X-original-commit: 2a16935ab86cb9635704ce03ef925b80954ca247
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/website_blog/views/website_blog_templates.xml

@@ -588,7 +588,7 @@
     <title t-esc="blog.name"/>
     <link t-att-href="'%s/blog/%s' % (base_url ,blog.id)"/>
     <id t-esc="'%s/blog/%s' % (base_url, blog.id)"/>
-    <updated t-esc="str((posts[0] if posts else blog).post_date).replace(' ', 'T') + 'Z'"/>
+    <updated t-esc="str(posts[0].post_date).replace(' ', 'T') + 'Z' if posts else ''"/>
     <entry t-foreach="posts" t-as="post">
         <title t-esc="post.name"/>
         <link t-att-href="'%s%s' % (base_url, post.website_url)"/>