Allow adding empty ROIs

[mathiser]

Dear rt-utils devs, Does the dicom format allow adding empty ROIs? This would be extremely useful to be able to: When a template rtstruct is made, our deep learning segmentation model does not always predict something for all labels. Thus, we just need to remember to add them manually later. With the possibility to add empty ROIs this problem is solved and making rt-utils even more useful.

Mathis