luci-base: fix ubus luci.getConntrackHelpers call with firewall4
luci-base: fix ubus luci.getConntrackHelpers call with firewall4

Fallback to firewall4's helper list if the fw3 one cannot be loaded.
Fixes broken zone configuration when firewall4 is installed as backend.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

openwrt/luci,openwrt/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,hnyman/luci,openwrt/luci,hnyman/luci,openwrt/luci,openwrt/luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,hnyman/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci