Throw sane error if one of security groups is null

"vararg" methods for both `NovaTemplateOptions` and `TemplateOptions` does not check values for nullness, resulting in `NullPointerException: null` from `ImmutableSet.copyOf`.
As @jdaggett said: please add `{ ... }`. Also:

* why are **empty** group names not allowed?
* we can remove the `checkNotNull` in the next line