[json] Allow null for string values

[feliwir]

See https://dicom.nema.org/dicom/2013/output/chtml/part18/sect_F.2.html in Section F.2.5

Based on #459

[Enet4]

Well, this indeed works! I will approve and merge while leaving these notes on potential future changes:

• There is an extra allocation from Vec<Option<Value>> to Vec<Value>, but this also happens in a few other places as well. Maybe someone could find some time to fine-tune these later.
• This takes care of the conversion from DICOM JSON to in-memory representations, but, thinking reciprocally, I suspect that empty values will still be encoded to DICOM JSON as empty strings with the current implementation. This is worth taking care of in a new pull request.

Thanks again for your contribution @feliwir. 👍