The test `TrustedXForwarderProxiesUnknownHostnameFailureTest` sometimes fail in CI due to a NPE in `ForwardedProxyHandler`. This shows an actual bug: per the documentation, `io.vertx.core.dns.DnsClient.lookup()` may succeed with a `null` value when no record was found. The `ForwardedProxyHandler` ignores the possibility of a `null` result, which this commit fixes. We deal with a `null` result just like with a failure, because it's equivalent to a NXDOMAIN error.

Fixes #36656