Make enumerate accept @Nullable Thread[] (ditto ThreadGroup).

[cpovirk]

Since the method only writes to the array, it is fine with either a
nullable element type or a non-nullable element type. (I'm operating
under the usual assumption that tools treat arrays as covariant. For
purposes of Checker Framework users who set -AinvariantArrays, we
could consider using @PolyNull instead in eisop.)

This change is prompted by a Kotlin caller that passes an array of
Thread? because Kotlin won't allow an uninitialized array of Thread.
In Java, I would expect most users to allocate an uninitialized array of
[@NonNull] Thread and pass that.

(This is a pretty weird API, though, since it requires you to pass in an
array that you pre-size for the number of threads you expect and so you
might overshoot and end up (I assume; the docs are not clear) with nulls
in the result....)

[wmdietl]

Thanks, looks good!
Using @PolyNull for EISOP sounds reasonable, too. The javadoc makes it sound like only the non-null threads are written in the array, so any remaining elements should stay untouched. So if an array with only non-null elements is passed in, all elements should stay non-null.