Handle case where `mac_list` is `{}` in Switch Status API response

[angristan]

Fix for https://github.com/trazfr/freebox-exporter/issues/3:

collector.go:346: json: cannot unmarshal object into Go struct field MetricsFreeboxSwitchStatus.result.mac_list of type []*struct { Mac string "json:\"mac\""; Hostname string "json:\"hostname\"" }

Full response for reference:

{
    "success": true,
    "result": [
        {
            "duplex": "full",
            "mac_list": [
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                }
            ],
            "name": "Ethernet 1",
            "link": "up",
            "id": 1,
            "mode": "100BaseTX-FD",
            "speed": "100",
            "rrd_id": "1"
        },
        {
            "duplex": "full",
            "mac_list": [
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                }
            ],
            "name": "Ethernet 2",
            "link": "up",
            "id": 2,
            "mode": "100BaseTX-FD",
            "speed": "100",
            "rrd_id": "2"
        },
        {
            "duplex": "full",
            "mac_list": [
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                },
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                },
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                },
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                },
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                }
            ],
            "name": "Ethernet 3",
            "link": "up",
            "id": 3,
            "mode": "1000BaseT-FD",
            "speed": "1000",
            "rrd_id": "3"
        },
        {
            "duplex": "full",
            "mac_list": [
                {
                    "mac": "xx:xx:xx:xx:xx:xx",
                    "hostname": "some device :)"
                }
            ],
            "name": "Ethernet 4",
            "link": "up",
            "id": 4,
            "mode": "100BaseTX-FD",
            "speed": "100",
            "rrd_id": "4"
        },
        {
            "duplex": "half",
            "name": "Freeplug",
            "link": "down",
            "id": 5,
            "mode": "10BaseT-HD",
            "speed": "10",
            "rrd_id": "freeplug"
        },
        {
            "duplex": "auto",
            "mac_list": {}, // ISSUE HERE
            "name": "Sfp lan",
            "link": "down",
            "id": 9999,
            "mode": "1000BaseT-FD",
            "speed": "1000",
            "rrd_id": "sfp_lan"
        }
    ]
}