goa CLI panics when run without any args

```shell
$ goa
goa is the code generation tool for the Goa framework.
Learn more at https://goa.design.

Usage:
  ...help text...

panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
main.main()
	/Users/andy/go/pkg/mod/goa.design/goa/v3@v3.18.2/cmd/goa/main.go:25 +0x520

```