prometheus-node-exporter-lua: wifi / wifi_stations error when wireless network is disabled

Maintainer: @champtar (me)
Environment: All

Description:

Wed Aug 14 16:46:53 2019 daemon.info prometheus-node-exporter-lua[11: /usr/lib/lua/prometheus-collectors/wifi.lua:16: bad argument #1 to 'type' (string expected, got nil)
Wed Aug 14 16:46:53 2019 daemon.info prometheus-node-exporter-lua[11: /usr/lib/lua/prometheus-collectors/wifi_stations.lua:29: bad argument #1 to 'type' (string expected, got nil)
Wed Aug 14 16:47:01 2019 daemon.info prometheus-node-exporter-lua[11: /usr/lib/lua/prometheus-collectors/wifi.lua:16: bad argument #1 to 'type' (string expected, got nil)
Wed Aug 14 16:47:01 2019 daemon.info prometheus-node-exporter-lua[11: /usr/lib/lua/prometheus-collectors/wifi_stations.lua:29: bad argument #1 to 'type' (string expected, got nil)

When interface is down, ifname is not present (which is normal)

root@OpenWrt:~# ubus call network.wireless status
{
	"radio0": {
		"up": false,
		"pending": false,
		"autostart": true,
		"disabled": true,
		"retry_setup_failed": false,
		"config": {
			"channel": "11",
			"hwmode": "11g",
			"path": "platform\/ar933x_wmac",
			"htmode": "HT20",
			"disabled": true
		},
		"interfaces": [
			{
				"section": "default_radio0",
				"config": {
					"mode": "ap",
					"ssid": "OpenWrt",
					"encryption": "none",
					"network": [
						"lan"
					],
					"mode": "ap"
				}
			}
		]
	}
}