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/alter_not_null_debug.test

@@ -75,7 +75,7 @@ SET DEBUG_SYNC='RESET';
 --echo #
 CREATE TABLE t1 (f VARCHAR(8) CHARACTER SET latin1 COLLATE latin1_swedish_ci)
 ENGINE=InnoDB;
-
+INSERT INTO t1 VALUES('ZERO');
 connection con1;
 SET DEBUG_SYNC = 'row_log_table_apply1_before SIGNAL scanned WAIT_FOR insert_done';
 send ALTER TABLE t1 MODIFY f VARCHAR(256) COLLATE latin1_german2_ci NOT NULL;