ddl: if the DDL is a noop, add logging to explain the reason

## Enhancement

```
create table tt (sessionid varchar(100) collate utf8mb4_unicode_ci not null, primary key (sessionid)) default charset=utf8mb4 collate=utf8mb4_unicode_ci;

-- The current **collate** of the table is already 'utf8mb4_unicode_ci', so this DDL operation is a noop.
alter table tt default character set utf8mb4 collate utf8mb4_unicode_ci;
```

There is no change for this query . And there are no record when execute 'admin show ddl jobs'.
In tidb.log , there is no logs except this ddl-query.

Actually can we log a message saying that the DDL statement does nothing for easy debugging?