Avoid double render in Avatars#show

After sending the conditional 304 response, we were still attempting to
render the response, which is unnecessary.

app/controllers/users/avatars_controller.rb

@@ -4,12 +4,8 @@ class Users::AvatarsController < ApplicationController
   before_action :set_user
 
   def show
-    fresh_when @user, cache_control: { max_age: 30.minutes, stale_while_revalidate: 1.week }
-
-    if @user.avatar.attached?
-      send_blob_stream @user.avatar
-    else
-      render_initials
+    if stale? @user, cache_control: { max_age: 30.minutes, stale_while_revalidate: 1.week }
+      render_avatar_or_initials
     end
   end
 
@@ -23,6 +19,14 @@ class Users::AvatarsController < ApplicationController
       @user = Current.account.users.find(params[:user_id])
     end
 
+    def render_avatar_or_initials
+      if @user.avatar.attached?
+        send_blob_stream @user.avatar
+      else
+        render_initials
+      end
+    end
+
     def render_initials
       render formats: :svg
     end