Hook is not called when arg is empty

[b4nst]

type tools []string

func (t *tools) BeforeResolve() error {
	fmt.Println("Hook")
	return nil
}

type Tool struct {
	Tools tools `arg:"" optional:"" help:"Filename of the tool to get information about. If empty, all tools from the toolbox are listed." type:"existingfile"` //nolint:lll // Struct tags are long.
}

BeforeResolve is only called if an arg is passed to the command. Is there a way to trigger it anyway (without adding a default value)?