Fix sortable when there's no record

app/models/mongoid/sortable.rb

@@ -23,7 +23,7 @@ module Mongoid
     def set_sort_number
       if self.new? && self.sort_number.nil?
         last_record = self.class.desc(:sort_number).first
-        if last_record.sort_number
+        if last_record && last_record.sort_number
           self.sort_number = (last_record.sort_number + 10) / 10 * 10
         else
           self.sort_number = 10