Json.toJson() return null directly

[lianghanzhen]

Hi, I have this issue: Json.toJson() return null value directly, I would like this method return "" rather than null. I figure out that I can handle this in getters, but I dont want to. Can I write a custom ObjectMapper or Serializer something like that to deal with it? Thanks
@Gissues:{"order":2.5641025641025084,"status":"backlog"}