Small cleanup of how RegionToEndpoint checks arguments.

I would take this out. This is a distraction and we could better solve this with a tool like error-prone than divert someone to wondering.. hmm what if that is not a String? Instead of focusing on the real code, there's a distraction.

I'd just note this potential snafu here: https://issues.apache.org/jira/browse/JCLOUDS-752
(ps take out @Nullable too).