apply "skipEmptyHistograms" tag also to data individually

Release notes

The "skipEmptyHistograms" tag of TQROOTPlotter now also applies to data histograms individually. Therefore, no markers are drawn if there are no data events in the histogram.

Details

As discussed on the mailing list: https://groups.cern.ch/group/qframework-users/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fqframework%2Dusers%2FLists%2FArchive%2Fdon%27t%20plot%20histogram%20markers%20for%20processes%20without%20events&FolderCTID=0x01200200BC70EC9B4B3D9548A2F698081355FE39