Unset/zero-value region should explicitly fail endpoint resolution

[lucix-aws]

There was a subtle change with either the introduction of EndpointResolverV2 or AuthSchemeResolver (I haven't deep-dived yet) - an empty region string on client config now incorrectly gets passed to endpoint resolution, which tends to result in nonsense hostnames like ram..amazonaws.com.

This should instead result in the "region" endpoint parameter being effectively unset/nil which will cause endpoint resolution to fail with an error that's actually useful.

There has been a ton of very warranted customer confusion around this recently, see the following:

• ram api not hitting the correct endpoint. #2501
• dial tcp: lookup sqs..amazonaws.com: no such host #2472
• ec2..amazonaws.com: no such host #2437

[github-actions]

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