runsc: don't create an empty network namespace if NetworkHost is set
runsc: don't create an empty network namespace if NetworkHost is set

With this change, we will be able to run runsc do in a host network namespace.

PiperOrigin-RevId: 246436660
Change-Id: I8ea18b1053c88fe2feed74239b915fe7a151ce34

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor