Fix #4508: Specific tag not showing a wiki.

Fix the wiki excerpt not appearing when searching for a tag that doesn't
exist in the tag list. This could happen if someone created a wiki for a
tag that has never been used on a post.

app/logical/post_sets/post.rb

@@ -24,9 +24,8 @@ module PostSets
     end
 
     def wiki_page
-      return nil unless tag.present? && tag.wiki_page.present?
-      return nil unless !tag.wiki_page.is_deleted?
-      tag.wiki_page
+      return nil unless normalized_query.has_single_tag?
+      @wiki_page ||= WikiPage.undeleted.find_by(title: normalized_query.tags.first.name)
     end
 
     def tag

test/functional/posts_controller_test.rb

@@ -96,6 +96,13 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           assert_response :success
         end
 
+        should "show the wiki excerpt for a wiki page without a tag" do
+          as(@user) { create(:wiki_page, title: "no_tag") }
+          get posts_path(tags: "no_tag")
+          assert_select "#show-excerpt-link", count: 1
+          assert_select "#excerpt", count: 1
+        end
+
         should "show a notice for a single tag search with a pending BUR" do
           create(:bulk_update_request, script: "create alias foo -> bar")
           get_auth posts_path(tags: "foo"), @user