The "standard" jclouds pattern would be `checkNotNull(networks, "networks")`, but in this case it's not doing the expected thing since a varargs argument cannot be `null`, it will be an empty array instead.

If we want to enforce this check, it probably should be:
```
checkArgument(networks.length > 0, "networks should not be empty");
return networks(ImmutableSet.copyOf(networks));
```