xAODFileMetaDataCnv: add run number to xAOD::FileMetaData
Store the run number in the xAOD::FileMetaData object to allow the autoconfiguration to find a meaningful value for the run number when working with all event-less input files.
This is to address the issue in finding the run number raised by @elmsheus in ATLASRECTS-6757.
Note: The capitalization is a hack to avoid name collisions in files that do contain events.