IR: make stub marker parameter for constructors nullable

In constructor stubs for default arguments, a marker parameter used
to have a non-null type, but null was passed as the corresponding
argument. This patch corrects the type.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/DefaultArgumentStubGenerator.kt

@@ -442,7 +442,7 @@ private fun IrFunction.generateDefaultsFunctionImpl(
         syntheticParameters += newFunction.valueParameter(
             syntheticParameters.last().index + 1,
             kConstructorMarkerName,
-            context.ir.symbols.defaultConstructorMarker.owner.defaultType
+            context.ir.symbols.defaultConstructorMarker.owner.defaultType.makeNullable()
         )
     } else if (context.ir.shouldGenerateHandlerParameterForDefaultBodyFun()) {
         syntheticParameters += newFunction.valueParameter(