Store empty connection_config in legacy CSR requests

Summary:
Versions of flipper from before 7th August 2018 don't return any response, so we don't store any connection_config.json file.
To keep the functionality the same as with up-to-date app versions, when this happens, store an empty config file ("{}").

Reviewed By: danielbuechele

Differential Revision: D9682885

fbshipit-source-id: fd580f6bba6b6b20135aa2c076be10e1eea0f8bc

xplat/Sonar/SonarWebSocketImpl.cpp

@@ -291,6 +291,8 @@ void SonarWebSocketImpl::sendLegacyCertificateRequest(folly::dynamic message) {
    ->fireAndForget(rsocket::Payload(folly::toJson(message)))
    ->subscribe([this, sendingRequest]() {
      sendingRequest->complete();
+     folly::dynamic config = folly::dynamic::object();
+     contextStore_->storeConnectionConfig(config);
      client_ = nullptr;
    });
 }