If the infra container has already terminated, `docker inspect` will report
pid 0. The path constructed using the pid to check the network namespace of
the process will be invalid. This commit changes docker to report an empty
path to stop kubenet from erroring out whenever TearDown is called on an
exited infra container.
This is not a fix for all the plugins, as some plugins may require the actual
network namespace to tear down properly.
fee4c9a7d9