fix: guard against empty firewall policy results to prevent panic

[Aprazor]

If the FortiOS API returns an empty array (no VDOMs or permission
error), accessing ps4[0] panics with index out of range.

[bastischubert]

Thanks for spotting this! The panic is real, but we'd prefer to fix the root cause: probeFirewallPolicies discards *TargetMetadata (the _ parameter) and re-parses the version from ps4[0] — which is what forces the index access in the first place.

Other probes (bgp_neighbors.go, managed_switch.go, etc.) use meta.VersionMajor/VersionMinor directly. If this probe does the same, ps4[0] is never accessed for version info, the panic becomes impossible by design, and the guard isn't needed.

Would you be open to updating the PR to use meta instead? Happy to help if needed.

[bastischubert]

@Aprazor - does that sound ok for you to go the meta route instead?