Use ipv6 if available

src/freedombone-app-pihole

     setupVars=$piholeDir/setupVars.conf
     echo "piholeInterface=${PIHOLE_IFACE}" > ${setupVars}
     echo "IPv4_address=${IPv4_address}" >> ${setupVars}
-    echo "IPv6_address=${IPv6_address}" >> ${setupVars}
+    if [ ${#IPv6_address} -gt 0 ]; then
+        echo "IPv6_address=${IPv6_address}" >> ${setupVars}
+    fi
     echo "piholeDNS1=${PIHOLE_DNS1}" >> ${setupVars}
     echo "piholeDNS2=${PIHOLE_DNS1}" >> ${setupVars}