Use null value instead of "Missing property" string

Currently when requesting properties in a result block a `"property": "Missing property"` string value is returned for all instances in the result set that do not have this property. Can that be changed to null, as in `"property": null`? That would leave the result JSON intact and be a much more concise representation. The property could also be omitted completely from the respective entries in the result list.

Currently, if a property has "Missing property" as string value, one is not be able to tell from the results if the property has that string value or if the property is actually missing.