// simplified example
// Jackson version I use is `2.10.3` (the latest stable patch version)
import com.fasterxml.jackson.core.JsonGenerator
import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.databind.SerializerProvider
import com.fasterxml.jackson.databind.annotation.JsonSerialize
import com.fasterxml.jackson.databind.ser.std.StdSerializer

class NothingData {
    val data: Nothing? = null
}

@JsonSerialize(using = NothingDataWithSerializer.NothingDataSerializer::class)
class NothingDataWithSerializer {
    val data: Nothing? = null

    class NothingDataSerializer @JvmOverloads constructor(t: Class<NothingDataWithSerializer>? = null) : StdSerializer<NothingDataWithSerializer>(t) {
        override fun serialize(value: NothingDataWithSerializer, generator: JsonGenerator, provider: SerializerProvider) {
            generator.writeStartObject()
            generator.writeObjectField("data", null)
            generator.writeEndObject()
        }
    }
}

class IntData {
    val data: Int? = null
}

fun main() {
    val mapper = ObjectMapper()

    println(mapper.writeValueAsString(NothingData()))                // {}
    println(mapper.writeValueAsString(NothingDataWithSerializer()))  // {"data":null}
    println(mapper.writeValueAsString(IntData()))                    // {"data":null}
}