NUL terminator byte breaks fortinet firewall module

[efd6]

When an appliance sends NUL-terminated syslog lines via TCP, we end up with a NUL in the event.original and in the last structured data field as it's rendered into the document. The Fortinet Fortigate integration approaches this by gsubing the NUL away

  - gsub:
      field: syslog5424_sd
      pattern: "\u0000"
      replacement: ""

The same should happen in the filebeat module.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)