ThirdPartyResources doesn't support AllNamespaces

[JorritSalverda]

The following lines check whether a namespace is set for ThirdPartyResources:

https://github.com/ericchiang/k8s/blob/master/tprs.go#L105-L107

And since AllNamespaces is an empty string this leads to the error being returned. Kubectl supports --all-namespaces with my custom resources with scope: Namespaced fine though, so it seems superfluous to do this check.