Values cleared using the "x" control produce "null"

[jakubklimek]

Link to distribution schema produces "null", resulting in invalid JSON-LD, when the field is cleared by the "x" control:

      "http://purl.org/dc/terms/conformsTo": {
        "@id": null
      }

The same goes for all clearable fields such ase the distribution title, author names, contact details, etc.