slow query log is not logging when log_slow_filter is empty

Switched to MariaDB 5.5.29 from Percona SQL Server 5.1a and noticed empty slow query log.

It looks like it is logging only when log_slow_filter is set, but it is not logging when log_slow_filter is set empty string.

Having filter set is not resolving the issue, since optimized, but still slow queries are not logged.

log_slow_filter = 'admin,filesort,filesort_on_disk,full_join,full_scan,query_cache,query_cache_miss,tmp_table,tmp_table_on_disk'

Test case:

SET long_query_time=1;

SET log_slow_filter='';

SET GLOBAL slow_query_log=1;

SET GLOBAL log_output='TABLE';

SELECT SLEEP(2);

SELECT * FROM mysql.slow_log;