Fix findSubtypes returning wrong values in super edge cases.

It's simpler to look at sealedSubclasses directly than to determine isSealed, so I fixed it while I was at it.

Author: k163377

src/main/kotlin/com/fasterxml/jackson/module/kotlin/KotlinAnnotationIntrospector.kt

@@ -63,22 +63,15 @@ internal class KotlinAnnotationIntrospector(private val context: Module.SetupCon
      * Subclasses can be detected automatically for sealed classes, since all possible subclasses are known
      * at compile-time to Kotlin. This makes [com.fasterxml.jackson.annotation.JsonSubTypes] redundant.
      */
-    override fun findSubtypes(a: Annotated): MutableList<NamedType>? {
-
-        val rawType = a.rawType
-        if (rawType.isKotlinClass()) {
-            val kClass = rawType.kotlin
-            if (kClass.isSealed) {
-                return kClass.sealedSubclasses
-                    .map { NamedType(it.java) }
-                    .toMutableList()
-            }
+    override fun findSubtypes(a: Annotated): MutableList<NamedType>? = a.rawType
+        .takeIf { it.isKotlinClass() }
+        ?.let { rawType ->
+            rawType.kotlin.sealedSubclasses
+                .map { NamedType(it.java) }
+                .toMutableList()
+                .ifEmpty { null }
         }
 
-        return null
-
-    }
-
     private fun AnnotatedField.hasRequiredMarker(): Boolean? {
         val byAnnotation = (member as Field).isRequiredByAnnotation()
         val byNullability =  (member as Field).kotlinProperty?.returnType?.isRequired()