After looking at GitHub for a solution I consulted the very helpful people in #LibreNMS at FreeNode (IRC) and got this solution.
Add this line in you config.php:
$config['os']['fortigate']['empty_ifdescr'] = 1;
And guess what, it works!
Why does LibreNMS behave like this? Well, it’s because LibreNMS assumes that there should be a “ifdescr” in the SNMP answer from the Firewall, while Fortigate has removed this in order to be compliant with an SNMP RFC that says that there should be no ifdescr if there is no description text for the interface. At least that was the information I was able to gather from the conversation on IRC and github/Internet!