use display_name to avoid name rendering as None

bookwyrm/views/rss_feed.py

@@ -216,7 +216,7 @@ def link(self, obj):
 
     def title(self, obj):
         """title of the rss feed entry"""
-        return _(f"{obj.user.name}’s {obj.name} shelf")
+        return _(f"{obj.user.display_name}’s {obj.name} shelf")
 
     def items(self, obj):
         """the user's activity feed"""
@@ -234,5 +234,5 @@ def description(self, obj):
         """description of the shelf including the shelf name and user."""
         # if there's a description, lets add it. Not everyone puts a description in.
         if desc := obj.description:
-            return _(f"{obj.user.name}’s {obj.name} shelf: {desc}")
+            return _(f"{obj.user.display_name}’s {obj.name} shelf: {desc}")
         return _(f"Books added to {obj.user.name}’s {obj.name} shelf")