Do not write for null update (properly fix MySQL Bug#29157)

monty noted that InnoDB takes a lot of time to perform null updates. The reason is that even though an empty update vector was created, InnoDB will go on to write undo log records and update the system columns DB_TRX_ID and DB_ROLL_PTR in the clustered index, and of course write redo log for all this.

This could have been fixed properly when MySQL Bug #29157 was addressed over 10 years ago.