fix(http): allow server_list to satisfy missing server validation

[corporate-gadfly]

OpenVox recently tightened HTTP service creation checks so that a missing server setting now raises an error. That unintentionally broke valid configurations that rely on server_list instead of a single server.

Update Puppet::HTTP::Service.create_service to treat server_list as a valid source of server configuration when no explicit server argument is provided. This keeps the security-related no-default-server behavior intact while restoring compatibility for agents configured via server_list.

Checklist

I have:

• read the CONTRIBUTING.md document
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• tested this code
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• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[corporate-gadfly]

I don't know Ruby that well, so I would prefer to use #659, if possible.

[silug]

Unfortunately I think this will still fail on systems with use_srv_records and srv_domain set.