MDEV-23805 Make Online DDL to Instant DDL when table is empty

- In ha_innobase::prepare_inplace_alter_table(), InnoDB should
check whether the table is empty. If the table is empty then
server should avoid downgrading the MDL after prepare phase.
It is more like instant alter, does change only in dicationary
and metadata.

- Changed few debug test case to make non-empty DDL table

mysql-test/suite/innodb/r/innodb-index-online.result

@@ -509,6 +509,7 @@ DROP TABLE t1;
 # MDEV-13205 assertion !dict_index_is_online_ddl(index) upon ALTER TABLE
 #
 CREATE TABLE t1 (c VARCHAR(64)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES('foo');
 SET DEBUG_SYNC = 'row_log_apply_before SIGNAL t1u_created WAIT_FOR dup_done';
 ALTER TABLE t1 ADD UNIQUE(c);
 connection con1;