rgw: Report empty endpoints as error instead of crashing

[adamemerson]

Empty endpoints are a misconfiguration, but should be rpeorted rather than crashing.

Checklist

• Tracker (select at least one)
  • References tracker ticket
  • Very recent bug; references commit where it was introduced
  • New feature (ticket optional)
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• Component impact
  • Affects Dashboard, opened tracker ticket
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  • No impact that needs to be tracked
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  • No doc update is appropriate
• Tests (select at least one)
  • Includes unit test(s)
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  • No tests

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[adamemerson]

@cbodley https://pulpito.ceph.com/aemerson-2022-01-20_18:59:04-rgw-wip-53941-distro-basic-smithi/