Segmentation that contains 0 only should not crash the converter

[fedorov]

If input JSON contains only label id "0", there should be an error. If image does not contain any labels that are passed in JSON there should be an error.

I do not want to force users to always define semantics of 0. We need to have a way to communicate that some slices do not contain anything of value (otherwise, how would we decide which slices to skip during conversion?). Right now, 0 in the input image effectively communicates that those slices do not have anything. If background needs to be encoded, it will have to be non-zero label.

If there is a need to explicitly encode certain segment as "Background", it would have to have a non-zero value in the image. Probably, the right code for that would be "Background" from this table: https://dicom.nema.org/medical/dicom/current/output/chtml/part16/sect_CID_7165.html.