Add test case to serialize Nothing?

wrt #314

Author: k163377

src/test/kotlin/com/fasterxml/jackson/module/kotlin/test/github/GitHub314.kt

@@ -0,0 +1,29 @@
+package com.fasterxml.jackson.module.kotlin.test.github
+
+import com.fasterxml.jackson.databind.MapperFeature
+import com.fasterxml.jackson.module.kotlin.jsonMapper
+import com.fasterxml.jackson.module.kotlin.kotlinModule
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class GitHub314 {
+    // Since Nothing? is compiled as a Void, it can be serialized by specifying ALLOW_VOID_VALUED_PROPERTIES
+    data object NothingData {
+        val data: Nothing? = null
+    }
+
+    @Test
+    fun test() {
+        val expected = """{"data":null}"""
+
+        val withoutKotlinModule = jsonMapper { enable(MapperFeature.ALLOW_VOID_VALUED_PROPERTIES) }
+        assertEquals(expected, withoutKotlinModule.writeValueAsString(NothingData))
+
+        val withKotlinModule = jsonMapper {
+            enable(MapperFeature.ALLOW_VOID_VALUED_PROPERTIES)
+            addModule(kotlinModule())
+        }
+
+        assertEquals(expected, withKotlinModule.writeValueAsString(NothingData))
+    }
+}