Fixed NPE when Server has no Image (JCLOUDS-558)

Fixes: https://issues.apache.org/jira/browse/JCLOUDS-558
Follow up to: https://github.com/jclouds/jclouds/pull/414
I'm okay with it the way it is. I think it's more clear that way. The more common case is what's being tested, as it should be.