fix return null iptables.append

return null in iptables.append,fix it
```
salt 'node1' iptables.append filter INPUT rule='-s 0.0.0.0 -p tcp --dport 80 -j ACCEPT' 
node1:
    True
salt 'node1' iptables.append filter INPUT rule='-s 0.0.0.0 -p tcp --dport 80 -j ACCEPT4'
node1:
    False
```