`groupsOfVarying` behaves inconsistently with groups of length `0`

```elm
> import List.Extra as List
> List.groupsOfVarying [0, 1] ["a"]
[[],["a"]] : List (List String)
> List.groupsOfVarying [1, 0] ["a"]
[["a"]] : List (List String)
```

I would expect the second expression to evaluate to `[["a"], []]`. But at the very least, I would expect the two to be symmetrical!