Fix assert setting sys variables for interfaces when none are detected

I came across this bug when I added *all* my interfaces (except lo) to /var/cfengine/inputs/ignore_interfaces.rx. What happened is simple, running the agent output this:

```
cf-agent: env_context.c:1425: EvalContextVariablePut: Assertion `value' failed.
```

A bit of digging showed that the functions to put variables ```assert``` that variable values are not empty, but the interface detection loop doesn't worry if it didn't find any interfaces to add. A few ifs fix that.

Please let me know if this is not a suitable approach (so many ifs look ugly to me, but hey...), and I can work on another way round.