Kotlin Klaxon cannot deserialize empty object map values as JsonObject

[schani]

Summary

The Kotlin/Klaxon renderer can generate map values as JsonObject for empty object shapes, but Klaxon fails to deserialize {} into JsonObject in that position.

Reproduction

This was found while adding the regression fixture in #2877:

{
  "mission_specs": {
    "1": { "objectives": { "2": { "rewards": { "3": {}, "5": {} } } } },
    "4": { "objectives": { "6": { "rewards": {} } } }
  }
}

The generated Kotlin contains:

val rewards: Map<String, Reward>
typealias Reward = JsonObject

Running the Kotlin/Klaxon fixture fails with:

KlaxonException: Couldn't find a suitable constructor for class JsonObject to initialize with {}

Expected behavior

The generated Kotlin/Klaxon code should deserialize empty object values in maps successfully, or represent that shape in a way Klaxon can handle.

Notes

#2877 is skipping its new fixture for Kotlin/Klaxon so the unrelated core regression fix can pass CI. Kotlin-Jackson did not hit this failure.