Added missing member initialization in `JSON::WriterProxy`.
Added missing member initialization in `JSON::WriterProxy`.

Review: https://reviews.apache.org/r/71522

reneploetz/mesos,kaysoky/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos