Gracefully handle when `iptables-save` isn't installed when retrieving instances

When using this module and purge the resources as documented, e.g.:
```puppet
resources { 'firewall':
  purge => true,
}
```
You'll see the following error when the `iptables-save` command isn't present:
```
Resources[firewall]: Failed to generate additional resources using 'generate': Command iptables_save is missing
```
This problem stems from the `self.instances` method on the `iptables` provider failing to take into account that the command may not exist and have a `nil` result.