[alts] Downgrade log level when handshaker service recv_buffer is null. (#37962)

This log can be hit under normal-ish circumstances, e.g. if the handshaker service fails to respond or is unreachable. For that reason, it should not be an error log.

Closes #37962

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37962 from matthewstevenson88:downgrade-log 36d4f006a0
PiperOrigin-RevId: 687986132

src/core/tsi/alts/handshaker/alts_handshaker_client.cc

@@ -233,7 +233,7 @@ void alts_handshaker_client_handle_response(alts_handshaker_client* c,
     return;
   }
   if (recv_buffer == nullptr) {
-    LOG(ERROR)
+    VLOG(2)
         << "recv_buffer is nullptr in alts_tsi_handshaker_handle_response()";
     handle_response_done(
         client, TSI_INTERNAL_ERROR,