A side effect of the MDEV-24589 bug fix is that if
FLUSH TABLE...FOR EXPORT is initiated before the history of an
earlier DROP INDEX operation has been purged, then the data file
will contain allocated pages that belonged to the dropped indexes.
These pages would never be freed after a subsequent IMPORT TABLESPACE.

We will work around this regression by making IMPORT TABLESPACE
tolerate pages that refer to an unknown index.