trx_t::commit_empty(): A special case of transaction "commit" when
the transaction was actually rolled back or the persistent undo log
is empty. In this case, we need to change the undo log header state to
TRX_UNDO_CACHED and move the undo log from rseg->undo_list to
rseg->undo_cached for fast reuse. Furthermore, unless this is the only
undo log record in the page, we will remove the record and rewind
TRX_UNDO_PAGE_START, TRX_UNDO_PAGE_FREE, TRX_UNDO_LAST_LOG.

Even though it is not going to be written persistently anywhere,
we will invoke trx_sys.assign_new_trx_no(this), so that in the test
innodb.instant_alter everything will be purged as expected.

trx_t::write_serialisation_history(): Renamed from
trx_write_serialisation_history(). If there is no undo log,
invoke commit_empty().

trx_purge_add_undo_to_history(): Simplify an assertion and remove a
comment. This function will not be invoked on an empty undo log
anymore.

trx_undo_header_create(): Add a debug assertion.

trx_undo_mem_create_at_db_start(): Remove a duplicated assignment.