JSON null is converted to "null"

Steps to reproduce:

1. Create an item prototype that uses JSON path preprocessing to extract a field. This field can either be a JSON string or JSON null.
2. Add a trigger prototype with the following expression: 

   length(last(/BackupPC/backuppc.host[{#HOST},error]))>0 

1. Monitor the template via a Zabbix Proxy
2. Since Zabbix Proxy 6.0.12 a JSON null is converted to the JSON string "null" causing the trigger to fire.

Expected:

• JSON null should not be converted to the string "null"