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core: fix scoped dispatchers clobbering the global default (#2065)
## Motivation PR #2001 introduced --- or rather, _uncovered_ --- a bug which occurs when a global default subscriber is set *after* a scoped default has been set. When the scoped default guard is dropped, it resets the thread-local default cell to whatever subscriber was the default when the scoped default was set. This allows nesting scoped default contexts. However, when there was *no* default subscriber when the `DefaultGuard` was created, it sets the "previous" subscriber as `NoSubscriber`. This means dropping a `DefaultGuard` that was created before any other subscriber was set as default will reset that thread's default to `NoSubscriber`. Because #2001 changed the dispatcher module to stop using `NoSubscriber` as a placeholder for "use the global default if one exists", this means that the global default is permanently clobbered on the thread that set the scoped default prior to setting the global one. ## Solution This PR changes the behavior when creating a `DefaultGuard` when no default has been set. Instead of populating the "previous" dispatcher with `NoSubscriber`, it instead leaves the `DefaultGuard` with a `None`. When the `DefaultGuard` is dropped, if the subscriber is `None`, it will just clear the thread-local cell, rather than setting it to `NoSubscriber`. This way, the next time the cell is accessed, we will check if a global default exists to populate the thread-local, and everything works correctly. As a side benefit, this also makes the code a bit simpler! I've also added a test reproducing the bug. This PR is against `v0.1.x` rather than `master`, because the issue does not exist on `master` due to other implementation differences in v0.2. We may want to forward-port the test to guard against future regressions, though. Fixes #2050
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