Increase resilience if lookup var comes back with nil object

This blows up on Windows 2003 as no interfaces show up in Facter.

has_interface.pp
```
$a = '192.168.0.15'
$o = has_interface_with('ipaddress', $a)
notice(inline_template('has_interface_with is <%= @o.inspect %>'))
```
puppet apply yields
```
Error: undefined method `split' for nil:NilClass at C:/has_interface.pp:2 on node xran6l3uabwistu.mydomain.com
Wrapped exception:
undefined method `split' for nil:NilClass
Error: undefined method `split' for nil:NilClass at C:/has_interface.pp:2 on node xran6l3uabwistu.mydomain.com
```