Event correlation will not work with empty tag values and Oracle database

Create a trigger with event correlation ('OK event closes' is set to 'All problems if tag values match').

The event correlation will not work if the matching tag values are empty - Zabbix would try to match tag values with empty strings, while Oracle stores them as NULL values.

Similar problem is with global event correlation 'event tag pair' and 'old event tag value' conditions.

The DBadd_str_condition_alloc() must correctly handle empty values when working with Oracle database and correlation_condition_add_tag_match() must use ZBX_SQL_STRVAL_EQ() macro for tag value comparison.