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/t/innodb-index-debug.test

@@ -96,6 +96,7 @@ drop table t1;
 # Log file creation failure.
 create table t1(k1 int auto_increment primary key,
 k2 char(200),k3 char(200))engine=innodb;
+INSERT INTO t1 VALUES(1, "test", "test");
 SET DEBUG_SYNC= 'row_merge_after_scan
 SIGNAL opened WAIT_FOR flushed';
 send ALTER TABLE t1 FORCE, ADD COLUMN k4 int;
@@ -122,6 +123,7 @@ drop table t480;
 --echo #
 
 CREATE TABLE t1 (j INT UNIQUE, i INT UNIQUE) ENGINE=InnoDB;
+INSERT INTO t1 VALUES(2, 2);
 --connect (con1,localhost,root,,test)
 SET DEBUG_SYNC='row_log_table_apply1_before SIGNAL built WAIT_FOR log';
 --send