br: log status json report missing status

[BornChanger]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

run br log stats --json

2. What did you expect to see? (Required)

log task status is reported in the json format output

3. What did you see instead (Required)

task status is missing

4. What is your TiDB version? (Required)