Fix query of values missing from first image in series
Fix query of values missing from first image in series

When the first images was missing an attribute that was being
queried for in vtkDICOMDirectory, then that attribute would be
reported as missing from all images in the series, even if some
of the images had the attribute.

dgobbi/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom