application/luci-app-firewall: fix null access

Fixes: 48086e1c ("luci-app-firewall: Add ipset field to snats")
Fixes: d0d891c2 ("luci-app-firewall: Add ipset field to forwards (redirects)")
Fixes: f407a013

 ("luci-app-firewall: Add ipset field to rules")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>