Enable #nullable for some of the disposable types.

[bartdesmet]

First few #nullable introductions for disposable types. Note that Disposable properties on 3 of these types are now marked as ? because they can return null.

More to come for the composite variants of disposables. The rub with all of these (and the query operators after inlining disposable logic everywhere) is that all the helper methods use ref parameters and often need to change to IDisposable? to be callable, but then the call site does not have a guarantee about assignment. So, for now, we're stuck with these helpers remaining null-oblivious.