PERF: Avoid hitting DB when fetching draft sequence of bot user.

app/models/draft_sequence.rb

@@ -23,6 +23,8 @@ class DraftSequence < ActiveRecord::Base
     user_id = user
     user_id = user.id unless user.is_a?(Integer)
 
+    return nil if user_id < 0
+
     # perf critical path
     r, _ = DB.query_single('select sequence from draft_sequences where user_id = ? and draft_key = ?', user_id, key)
     r.to_i