FIX: set microdata schema for topic on missing first post (#25195)

Some attributes of the microdata schema `DiscussionForumPosting` are rendered in the context of the first post.
Ensure these attributes are also set if the first post is not part of the current view.

app/views/topics/show.html.erb

@@ -51,7 +51,7 @@
         <% end %>
       </div>
 
-      <% if @topic_view.prev_page %>
+      <% if @topic_view.posts&.first && !@topic_view.posts.first.is_first_post? %>
         <span itemprop='author' itemscope itemtype="http://schema.org/Person">
           <meta itemprop='name' content='<%= @topic_view.topic.user.username %>'>
           <link itemprop='url' href='<%= Discourse.base_url %>/u/<%= @topic_view.topic.user.username %>'>