ConstexprEvaluator blows up on empty array

The following DSLX causes a boom (out-of-bounds vector access) in the constexpr evaluator:

```
type Block = u8[4][4];

fn foo() {
let bar = Block:[];
()
}
```

1. The ConstexprEvaluator should not explode ever
2. Parsing should flag this as an error.