JSON "null" matches empty string in LLD filters

Suppose we send a JSON like the following to Zabbix server:

{
    "data": [
        {
            "{#MACRO}": "macro",
            "{#FILTER}": null
        }
    ]
}

If we filter by {#FILTER}, then "null" will be processed like an empty string and will match regular expression "^$". Instead, the filter should fail.

The reason for the behavior above is that our JSON parser currently parses JSON null as an empty string.