bug: Allow null end parameter in std.slice / array indexing

refs: https://github.com/CertainLach/jrsonnet/issues/176

```jsonnet
jsonnet -e "std.slice(std.range(1,4), 0, null, 2)"
[
   1,
   3
]
```

In sjsonnet got :

```scala
sjsonnet.Error: Wrong parameter type: expected Int, got null
    at [std.slice].(:1:10)

```

, but it works fine in official jsonnet

```json
[
  1,
  3
]
```