JSON.toJSONString()空字段不忽略修改的问题

public static void main(String[] args) {
    DossierApply dossierApply = new DossierApply();
    String s = JSON.toJSONString(dossierApply, valueFilter);
    System.out.println(s);
} 
private static ValueFilter valueFilter = (o, s, o1) -> o1 == null ? "" : o1;

    public Long getuId() {
        return uId;
    } 
    public void setuId(Long uId) {
        this.uId = uId;
    }

    public void setUId(Long uId) {
        this.uId = uId;
    } 
    public Integer getDtId() {
        return dtId;
    }