mtr_t::commit() unnecessarily acquires log_sys.mutex when writing no log

In mtr_t::commit(), we acquire log_sys.mutex even when no redo log has been generated.

Before log_sys.lsn was changed to be std::atomic in MariaDB 10.5.2, acquiring the mutex was necessary. Likewise, before MDEV-12227 in MariaDB 10.5.9, pages of temporary tablespace were being added to the buf_pool.flush_list. After those changes, there should be no reason to acquire log_sys.mutex when committing a mini-transaction that is not changing any persistent data.