AthenaMonitoring: allow empty ToolHandleArray in AthMonitorAlgorithm::getGroup

Hello,

As discussed in ATR-30693, we have a trigger ART test that runs with monitoring algorithms but without monitoring tools. Currently, this is not a supported case for AthMonitorAlgorithm::fill( const std::string& groupName, MonVarVec_t&& variables ) as AthMonitorAlgorithm::getGroup issues FATAL error messages. However, this is a supported case for AthMonitorAlgorithm::fill( const ToolHandle<GenericMonitoringTool>& groupHandle, MonVarVec_t&& variables ) and Monitored::Group, nothing is done if we try to fill histograms while the tool handle is empty. With this MR, the first method behaves as the second method.

Cheers, Bertrand