[BUG] govc host.esxcli crashes when no arguments are specified

[Syuparn]

Describe the bug
When govc host.esxcli is executed without any arguments, govc crashes by runtime error.

To Reproduce
Steps to reproduce the behavior:

1. run command below to a vCenter

govc host.esxcli

Expected behavior
host.esxcli succeeds without any output (or return validation error that tells no arguments are specified).

Affected version

• govc 0.29.0

Screenshots/Debug Output
govc crashes by slice bounds out of range.

$ govc host.esxcli
panic: runtime error: slice bounds out of range [:-1]

goroutine 1 [running]:
github.com/vmware/govmomi/govc/host/esxcli.(*Command).Namespace(...)
        /home/runner/work/govmomi/govmomi/govc/host/esxcli/command.go:79
github.com/vmware/govmomi/govc/host/esxcli.(*Executor).CommandInfo(0x19b2bc0?, 0xc0000050c8?)
        /home/runner/work/govmomi/govmomi/govc/host/esxcli/executor.go:76 +0x4d5
github.com/vmware/govmomi/govc/host/esxcli.(*Executor).NewRequest(0x9?, {0xc0001163b0?, 0x27?, 0xc000166840?})
        /home/runner/work/govmomi/govmomi/govc/host/esxcli/executor.go:110 +0x88
github.com/vmware/govmomi/govc/host/esxcli.(*Executor).Run(0xc000166840?, {0xc0001163b0?, 0x19bda98?, 0xc00011e090?})
        /home/runner/work/govmomi/govmomi/govc/host/esxcli/executor.go:153 +0x27
github.com/vmware/govmomi/govc/host/esxcli.(*esxcli).Run(0xc000108e30, {0x19bda98?, 0xc00011e090?}, 0xc000156360)
        /home/runner/work/govmomi/govmomi/govc/host/esxcli/esxcli.go:89 +0x85
github.com/vmware/govmomi/govc/cli.Run({0xc0001163b0, 0x1, 0x1})
        /home/runner/work/govmomi/govmomi/govc/cli/command.go:165 +0x32d
main.main()
        /home/runner/work/govmomi/govmomi/govc/main.go:114 +0x4f