After further discussion with the team this morning, we decided that the proposed "dummy chunk" workaround is unacceptable (the interpreter may end up trying free it when shutting down a thread).

Allowing the tstate->datastack_* members to be NULL is very easy on our end, and even simplifies thread creation a tad (since we can defer the allocation of the datastack until the first frame is actually needed).

I'll have a PR up in a few minutes.