Steps to reproduce:

1. Create a host
2. Create an item

1. 1. Select "Database monitor" as a type
   2. Select "db.odbc.select[<unique short description>,<dsn>]" key
   3. Remove both parameters (frontend states that these are optional)
   4. Fill other required fields

Result:

Frontend creates an item without any error message.

When Zabbix starts to monitor created item, following line appears in Zabbix server's log:

 28861:20190205:110746.032 item "Host:db.odbc.select" became not supported: Invalid number of parameters.

Expected:

Even if description is optional, dsn parameter is mandatory. Frontend should not mark it as optional.

https://www.zabbix.com/documentation/current/manual/config/items/itemtypes/odbc_checks states that description field is mandatory as well (i.e., no "<>" symbols there).

Item key "db.odbc.discovery" is confusing as well. Documentation (https://www.zabbix.com/documentation/4.0/manual/discovery/low_level_discovery/sql_queries) states that both parameters are optional.