ArgumentsBuilder: make SOCKS proxy authorization optional, add null checks if proxy url is set

Suppose developer wants to use the SOCKS proxy
```Shell
ssh -D 1080 user@host
```

Examine the following code:
```Java
final DesiredCapabilities capabilities = DesiredCapabilities.phantomjs();
final Proxy proxy = new Proxy();
proxy.setHttpProxy(""); // Workaround: httpProxy defaults to null -> NullPointerException @ PhantomJSDriverService:465 [if (!proxy.getHttpProxy().isEmpty()) {          //< HTTP proxy]
proxy.setSocksProxy("127.0.0.1:8000");
capabilities.setCapability(CapabilityType.PROXY, proxy);
final WebDriver webDriver = new PhantomJSDriver(capabilities);
```

httpProxy must be set to empty string
also there is no way to not use the SOCKS authorization
current implementation appends invalid
```Shell
--proxy-auth=null:null
```