Add EmptyProducer class

Kernel/LHCbAlgs/include/LHCbAlgs/EmptyProducer.h

+/*****************************************************************************\
+* (c) Copyright 2000-2021 CERN for the benefit of the LHCb Collaboration      *
+*                                                                             *
+* This software is distributed under the terms of the GNU General Public      *
+* Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
+*                                                                             *
+* In applying this licence, CERN does not waive the privileges and immunities *
+* granted to it by virtue of its status as an Intergovernmental Organization  *
+* or submit itself to any jurisdiction.                                       *
+\*****************************************************************************/
+#include "LHCbAlgs/Producer.h"
+
+template <typename Out>
+class EmptyProducer final : public LHCb::Algorithm::Producer<Out()> {
+public:
+  EmptyProducer( const std::string& name, ISvcLocator* pSvcLocator )
+      : LHCb::Algorithm::Producer<Out()>{name, pSvcLocator, {"Output", ""}} {}
+
+  Out operator()() const override { return {}; }
+};