Zabbix

........S. [ZBXNEXT-4108] replace NULL with "" in add_event() error field

Explicitly pass "" instead of NULL when calling the add_event()
function. the previos behaviour relied on the database access
layer setting "" if a field is NULL.
This could be dangerous, as the two functionalities are detached
enough for causing potential problems in the case the database
layer will be changed.
There is still an edge case where the name field is set as NULL,
but this should be an exception. A developer calling the
add_even()t function, MUST set the error field to "" if no errors
must be stored in the database.