Don't show word count if there isn't one

This mostly covers spotify pages, which use a custom reading time but
don't have words.

website/common/templates/common/content-details.html

@@ -15,7 +15,9 @@
     <div class="control">
       <div class="tags has-addons">
         <span class="tag is-dark"><i class="far fa-clock"></i></span>
-        <span class="tag is-light" title="{{ page.word_count }} words">{{ page.reading_time|naturaldelta }}</span>
+        <span class="tag is-light" {% if page.word_count %}title="{{ page.word_count }} words"{% endif %}>
+          {{ page.reading_time|naturaldelta }}
+        </span>
       </div>
     </div>
   {% endif %}

website/spotify/models.py

@@ -48,6 +48,10 @@ class SpotifyPlaylistPage(BaseContentPage):
             )
         )
 
+    @cached_property
+    def word_count(self) -> int:
+        return 0
+
     @cached_property
     def subtitle(self) -> str:
         return self.playlist_data["description"]