BUG: Fixed SH node creation if there are multiple series node for the same series UID

In some cases there might be multiple subject hierarchy nodes for the same DICOM series, for example if a series contains instances that load to different node types that cannot be simply added under one series folder node. This can happen if for one type the node corresponds to the series, but in the other to the instances, like segmentation nodes vs markup nodes under a folder node.

Fixed SubjectHierarchyGenericSelfTest that failed due to empty scene object in scene model

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24865 3bd1e089-480b-0410-8dfb-8563597acbee