xAODEventFormatCnv: Fix handling of rejected events.
If an event is rejected for an output stream, there won't be a DataHeader made for that stream. In EventFormatStreamHelperTool, handle the case of a missing DataHeader.
If an event is rejected for an output stream, there won't be a DataHeader made for that stream. In EventFormatStreamHelperTool, handle the case of a missing DataHeader.