Fail fast on empty round-robin address resolution

[hyperxpro]

Motivation:

PR #2229 changed round-robin mode to schedule the request timeout only once. As part of that change, the new-connection path assumes a successful DNS resolution always returns at least one address and unconditionally accesses the first entry. If a custom NameResolver incorrectly reports success with an empty address list, this results in an IndexOutOfBoundsException that is swallowed by Netty, leaving the request future incomplete and no request timeout scheduled. The request then hangs until the caller's own timeout expires.

Modification:

Guard the round-robin new-connection path against an empty resolved address list. If resolution succeeds with no addresses, fail the request immediately with an UnknownHostException containing the requested host instead of indexing into the list. Add a test covering a custom resolver that returns an empty successful result and verify the request fails fast instead of hanging.

Result:

Round-robin requests now fail fast with a clear UnknownHostException when a resolver returns an empty address list, avoiding a swallowed exception and a hung request. Existing behavior for valid resolvers and DEFAULT mode is unchanged.