Issue: Largest Series Product

In this exercise, there are two tests which don't seem to be correct:

```kotlin
@Test
fun reports1ForEmptyStringAndEmptyProduct() {
assertEquals(1, Series("").getLargestProduct(0))
}

@Test
fun reports1ForNonEmptyStringAndEmptyProduct() {
assertEquals(1, Series("123").getLargestProduct(0))
}

```

The requirement is: _Given a string of digits, calculate the largest product for a contiguous substring of digits of length n._

Is it logical to return the value 1 when the arguments don't allow to find a solution? It looks like those two tests should return an `IllegalArgumentException`:

- first case: contradicts _Given a string of digits_
- second case: contradicts _substring of digits of length n_

If this is indeed what the exercise should be, then it should be specified explicitly IMHO.