fixed a bug in code_gen.cpp when argument is empty.
fixed a bug in code_gen.cpp when argument is empty.

hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos