ha_innobase::check_if_supported_inplace_alter(): Do not invoke
innobase_table_is_empty() if the tablespace has been discarded.
That is, native ALTER TABLE in InnoDB will treat an empty table
in the same way as a tablespace whose tablespace has been discarded.
(Note: ALTER TABLE...ALGORITHM=COPY will fail if the tablespace
has been discarded.)

This fixes a crash that was introduced
in commit c7559747 (MDEV-19611).