Omitted variadic function parameter should be nil

### Example
```go
func(s ...string) {
    if s == nil {
        fmt.Println("s is nil")
    }
}()
```
With Go compiler `s == nil` ([playground](https://play.golang.org/p/G2jPMo5sT-0))
With GopherJS `s != nil` ([playground](https://gopherjs.github.io/playground/#/1jQnzf995r))

### Spec
https://golang.org/ref/spec#Passing_arguments_to_..._parameters