IpHelper does not detect invalid IP addresses

### Steps to reproduce the issue

```php
use Joomla\Utilities\IpHelper;

// Prepare SERVER array
$_SERVER['REMOTE_ADDR'] = 'fake';

// Prepare IpHelper singleton
IpHelper::setAllowIpOverrides(false);
IpHelper::setIp(null);

// Retrieve IP address
echo IpHelper::getIp();
```

### Expected result

Empty string

### Actual result

fake
