Handle empty array in the subscript operator

Addresses #2133 
That's just a flaw in the bytecode compiler, like the exception says it's "not yet implemented". If you want to dive in and fix it, that'd be great :)

It looks like the problem is that it's trying to box/unbox the `void` type, which I don't think is necessary since `void` has only a single value (the `null` value)