MDEV-23805 simplified the treatment of empty tables during ALTER TABLE,
which could prevent the scenarios that were previously reported and
fixed as MDEV-16131 and MDEV-24730.

With the MDEV-23805 fix, the statement
SET DEBUG_SYNC = 'now WAIT_FOR copied';
could occasionally time out, depending on timing.
Apparently, there was a race condition where purge could resume
(and empty the table) before ALTER TABLE got the chance to execute.
We must prevent the purge of history from running before
ALTER TABLE has started executing.