This fix tries to address the issue raised in 36139 where
ExitCode and PID does not show up in Task.Status.ContainerStatus
The issue was caused by `json:",omitempty"` in PID and ExitCode
which interprate 0 as null.
This is confusion as ExitCode 0 does have a meaning.
This fix removes `json:",omitempty"` in ExitCode and PID,
but changes ContainerStatus to pointer so that ContainerStatus
does not show up at all if no content. If ContainerStatus
does have a content, then ExitCode and PID will show up (even if
they are 0).
This fix fixes 36139.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
9247e09944