[EventEngine] Return error instead of assertion when hostname is empty in cf_engine (#35353)

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Closes #35353

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35353 from yijiem:fix-ios-dns-test f84d29a9db
PiperOrigin-RevId: 592339589

src/core/lib/event_engine/cf_engine/dns_service_resolver.cc

@@ -50,7 +50,14 @@ void DNSServiceResolverImpl::LookupHostname(
     });
     return;
   }
-  GPR_ASSERT(!host.empty());
+  if (host.empty()) {
+    engine_->Run([on_resolve = std::move(on_resolve),
+                  status = absl::InvalidArgumentError(absl::StrCat(
+                      "host must not be empty in name: ", name))]() mutable {
+      on_resolve(status);
+    });
+    return;
+  }
   if (port_string.empty()) {
     if (default_port.empty()) {
       engine_->Run([on_resolve = std::move(on_resolve),