Fix failbooru in /users/custom_style.css when custom_style is nil.

http://danbooru.donmai.us/users/custom_style.css failed when manually
requested by anonymous users, or by users who didn't have a style set.

app/logical/anonymous_user.rb

@@ -218,7 +218,7 @@ class AnonymousUser
   end
 
   def custom_style
-    nil
+    ""
   end
 
   def show_deleted_children?

app/views/users/custom_style.css.erb

@@ -1,4 +1,4 @@
-<% CurrentUser.user.custom_style.split(/\r\n|\r|\n/).each do |line| %>
+<% CurrentUser.user.custom_style.to_s.split(/\r\n|\r|\n/).each do |line| %>
   <% if line =~ /^@import/ %>
     <%= raw(line) %>
   <% else %>