An empty object should be deserialized as *the* Unit instance for a nullable Unit reference Type.

[schmist]

Describe the bug
Deserializing an empty object {} with reference Type Unit? leads to a new instance of kotlin.Unit instead of the singleton kotlin.Unit instance.

To Reproduce
assertSame(jacksonObjectMapper().readValue<Unit?>("{}"), Unit) or see submitted PR #519 with failing test.

Expected behavior
Deserialization to the kotlin.Unit singleton.

Versions
Kotlin: 1.5.31
Jackson-module-kotlin: 2.13.0
Jackson-databind: 2.13.0

Additional context

• Jackson deserializes kotlin.Unit singleton as new instances. #196
• When enabling singletonSupport it works as expected but I guess since it works for Unit without enabling this feature, it should work for Unit? too.