Render contracts in kotlinp, fix minor bug in kotlinx-metadata-jvm

libraries/kotlinx-metadata/jvm/ChangeLog.md

@@ -5,6 +5,7 @@
 - [`KT-31308`](https://youtrack.jetbrains.com/issue/KT-31308) Add module name extensions to kotlinx-metadata-jvm
 - [`KT-31338`](https://youtrack.jetbrains.com/issue/KT-31338) Retain "is moved from interface companion" property flag in kotlinx-metadata-jvm
     - Breaking change: JvmPropertyExtensionVisitor.visit has a new parameter `jvmFlags: Flags`
+- Correctly write "null" constant value in effect expression of a contract
 - Rename `desc` parameters to `signature` in JvmFunctionExtensionVisitor, JvmPropertyExtensionVisitor, JvmConstructorExtensionVisitor
 
 ## 0.0.5

libraries/kotlinx-metadata/src/kotlinx/metadata/impl/writers.kt

@@ -384,11 +384,10 @@ private fun writeEffectExpression(c: WriteContext, output: (ProtoBuf.Expression.
         }
 
         override fun visitConstantValue(value: Any?) {
-            @Suppress("UNUSED_VARIABLE") // force exhaustive when
-            val unused = when (value) {
+            when (value) {
                 true -> t.constantValue = ProtoBuf.Expression.ConstantValue.TRUE
                 false -> t.constantValue = ProtoBuf.Expression.ConstantValue.FALSE
-                null -> null
+                null -> t.constantValue = ProtoBuf.Expression.ConstantValue.NULL
                 else -> throw IllegalArgumentException("Only true, false or null constant values are allowed for effects (was=$value)")
             }
         }