core: fix scoped dispatchers clobbering the global default

[hawkw]

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