`WeakReference`, etc.: Avoid `@Nullable` on constructor parameter?

[cpovirk]

I suspect that we'll want to keep it, but I want to collect together related discussions:

• In Add nullability annotations to some JDK APIs. typetools/checker-framework#3029 (comment), we discussed that it's weird but that lots of people do pass null.
• I don't know if it came up there, but I think I learned later that new WeakReference<>(null) never gets enqueued, which actually seems fairly scary?
• Kotlin/Native has an (experimental) WeakReference whose constructor doesn't allow null—hence google/xplat@5cb35db, I assume.

[cpovirk]

(I would have to think about how much of the answer is "You should use Cleaner nowadays (and incidentally avoid SoftReference)"....)