This happens on disks which have never completed a SMART self-test before — the JSON field .ata_smart_data.self_test.passed is not present while the test is running.

Steps to reproduce:

1.  get new disk
2.  check in zabbix_smart plugin: 

   # /usr/sbin/zabbix_agent2  -t smart.disk.get["/dev/sdb","sat"]
   smart.disk.get[/dev/sdb,sat][s|...,"self_test_passed":true,...]
   

1. # /usr/sbin/zabbix_agent2  -t smart.disk.get["/dev/sdb","sat"]
   smart.disk.get[/dev/sdb,sat] [s|...,"self_test_passed":true,...]

Result:
See screenshot...
See log file...
See memory dump...
Expected:
See screenshot....
See attached patch file...