One of the features of `UnknownArrayHandle` is that it allows you to
query how many `Vec` components each value has without resolve the type
of the array. The functionality to implement this failed if you tried to
store an `ArrayHandle` that stored `Vec`-like objects with `Vec` sizes
that varied from value to value (i.e. an `ArrayHandleGroupVecVariable`).

Storing such an array in `UnknownArrayHandle` might not be the best
idea, but it should probably work. This change allows you to store such
an array. If you try to query the number of components, you will get 0.