Fix nil dereference in nodes list command.
Fixes a nil pointer dereference observed when listing nodes that have
not yet connected.
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb931a4]
goroutine 1 [running]:
github.com/juanfont/headscale/cmd/headscale/cli.glob..func8(0x13c93e0, 0xc0004c4220, 0x0, 0x2)
/go/src/headscale/cmd/headscale/cli/nodes.go:74 +0x364
github.com/spf13/cobra.(*Command).execute(0x13c93e0, 0xc0004c41e0, 0x2, 0x2, 0x13c93e0, 0xc0004c41e0)
/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:856 +0x2c2
github.com/spf13/cobra.(*Command).ExecuteC(0x13ca2e0, 0xc000497110, 0xe76416, 0x6)
/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:960 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
/go/pkg/mod/github.com/spf13/cobra@v1.1.3/command.go:897
main.main()
/go/src/headscale/cmd/headscale/headscale.go:89 +0x805
command terminated with exit code 2
```
fdcd3bb574