Change assert to conditional in sequence query.
Change assert to conditional in sequence query.

The assert would be triggered by an invalid query, and we don't
want an invalid query to potentially crash the program.  Also,
it's good to do a universal match check here (item of zero length).

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