Config does not accept null
Hi,

I installed the plugin in homebridge but get errors when homebridge is loading. I am using the Shelly 1 without username and password.

This is the error I get.
```
Load homebridge-shelly.Shelly

/homebridge/node_modules/homebridge-shelly/platform.js:127
      if (config.username && config.password) {
                 ^
TypeError: Cannot read property 'username' of null
    at new ShellyPlatform (/homebridge/node_modules/homebridge-shelly/platform.js:127:18)
    at Server._loadDynamicPlatforms (/usr/local/lib/node_modules/homebridge/lib/server.js:347:30)
    at Server.run (/usr/local/lib/node_modules/homebridge/lib/server.js:88:8)
    at module.exports (/usr/local/lib/node_modules/homebridge/lib/cli.js:45:10)
    at Object.<anonymous> (/usr/local/lib/node_modules/homebridge/bin/homebridge:17:22)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
```
This is my config:

```
{
    "bridge": {
        "name": "Homebridge",
        "username": "CC:22:3D:E3:CE:30",
        "port": 51826,
        "pin": "031-45-154"
    },
    "description": "Your description here",
    "accessories": [
        {
            "accessory": "RaspberryPiTemperature",
            "name": "RaspberryPi CPU Temperature",
            "updateInterval": 1000
        }
    ],
    "platforms": [
        {
            "platform": "eWeLink",
            "name": "eWeLink",
            "email": "myemail",
            "password": "mypassword",
            "imei": "977A80C0-EE3A-4B47-8B7C-93D2436D3BB7"
        },
        {
            "platform": "Sonoff",
            "name": "Sonoff",
            "devices": [
                {
                    "name": "Sonoff test",
                    "hostname": "192.168.1.110"
                },
                {
                    "platform": "Shelly",
                    "name": "Shelly",
                    "username": null,
                    "password": null
                }
            ]
        }
    ]
}


```

Can you help me fix this?