fix(wcth): match legacy TH w/ empty DNS reply (#546)

See https://github.com/ooni/probe/issues/1707#issuecomment-944322725

internal/cmd/oohelperd/internal/webconnectivity/dns.go

@@ -51,8 +51,13 @@ func dnsMapFailure(failure *string) *string {
 			// already checking for this specific error string.
 			s := webconnectivity.DNSNameError
 			return &s
-		case netxlite.FailureDNSNoAnswer,
-			netxlite.FailureDNSNonRecoverableFailure,
+		case netxlite.FailureDNSNoAnswer:
+			// In this case the legacy TH would produce an empty
+			// reply that is not attached to any error.
+			//
+			// See https://github.com/ooni/probe/issues/1707#issuecomment-944322725
+			return nil
+		case netxlite.FailureDNSNonRecoverableFailure,
 			netxlite.FailureDNSRefusedError,
 			netxlite.FailureDNSServerMisbehaving,
 			netxlite.FailureDNSTemporaryFailure:

internal/cmd/oohelperd/internal/webconnectivity/dns_test.go

@@ -31,7 +31,7 @@ func Test_dnsMapFailure(t *testing.T) {
 	}, {
 		name:    "no answer",
 		failure: stringPointerForString(netxlite.FailureDNSNoAnswer),
-		want:    stringPointerForString("dns_server_failure"),
+		want:    nil,
 	}, {
 		name:    "non recoverable failure",
 		failure: stringPointerForString(netxlite.FailureDNSNonRecoverableFailure),