Handle empty namespace consistently

[Random-Liu]

In #5, we decided to ignore empty network namespace.

However, this is not handled consistently across different network plugins:

• Plugins that ignore empty namespace and don't return error: bridge, ptp, ipvlan, macvlan, vlan;
• Plugins that return error if empty namespace is passed in: loopback, host-device.

Why don't we handle them consistently?

/cc @jingax10

[rosenhouse]

What’s the use case for empty namespace?

[Random-Liu]

It seems that there was some maintainer discussion before. #5

I think it is usually used when teardown network. Sometimes the network namespace is gone, but some resources are still not released, you need to run teardown to completely release all resources. For example, the ip checkpoint for host-local.

[Random-Liu]

This is also defined in the new CNI spec containernetworking/cni@23a5d8e