Problem
=======
When binlog is enabled, we do not save the gtid specified by
GTID_NEXT into GLOBAL@gtid_executed when committing an empty
transaction caused by a 'BEGIN' command. But we save the gtid
into GLOBAL@gtid_executed in the case when binlog is disabled.

Fix
===
When @@SESSION.GTID_NEXT == 'UUID:NUMBER', the command 'BEGIN' causes
an error 'ER_CANT_DO_IMPLICIT_COMMIT_IN_TRX_WHEN_GTID_NEXT_IS_SET'
inside an empty/a non-empty transaction regardless of being binlogless
or not, since it causes an implicit commit like other DDLs. And we do
not save the gtid specified by GTID_NEXT into GLOBAL@gtid_executed
in the case.