Don't skip absent args during function serialization.

So that we can recover an explicit null value correctly.

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/serialization/KonanIr.proto

@@ -94,7 +94,7 @@ message MemberAccessCommon {
     required TypeMap type_map = 3;
     optional IrExpression dispatch_receiver = 5;
     optional IrExpression extension_receiver = 6;
-    repeated IrExpression value_argument = 7;
+    repeated NullableIrExpression value_argument = 8;
 }
 
 message IrCall {
@@ -306,6 +306,10 @@ message IrExpression {
     required Coordinates coordinates = 3;
 }
 
+message NullableIrExpression {
+    optional IrExpression expression = 1;
+}
+
 
 /* ------ Declarations --------------------------------------------- */
 

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/serialization/SerializeIr.kt

@@ -195,6 +195,7 @@ internal class IrSerializer(val context: Context,
 
         call.descriptor.valueParameters.forEach {
             val actual = call.getValueArgument(it.index)
+            val argOrNull = KonanIr.NullableIrExpression.newBuilder()
             if (actual == null) {
                 // Am I observing an IR generation regression?
                 // I see a lack of arg for an empty vararg,
@@ -202,9 +203,9 @@ internal class IrSerializer(val context: Context,
                 assert(it.varargElementType != null ||
                     it.declaresDefaultValue())
             } else {
-                val argProto = serializeExpression(actual)
-                proto.addValueArgument(argProto)
+                argOrNull.expression = serializeExpression(actual)
             }
+            proto.addValueArgument(argOrNull)
         }
         return proto.build()
     }
@@ -809,8 +810,11 @@ internal class IrDeserializer(val context: Context,
 
     private fun deserializeMemberAccessCommon(access: IrMemberAccessExpression, proto: KonanIr.MemberAccessCommon) {
 
-        proto.valueArgumentList.mapIndexed { i, expr ->
-            access.putValueArgument(i, deserializeExpression(expr))
+        proto.valueArgumentList.mapIndexed { i, arg ->
+            val exprOrNull = if (arg.hasExpression())
+                deserializeExpression(arg.expression)
+            else null
+            access.putValueArgument(i, exprOrNull)
         }
 
         if (proto.hasDispatchReceiver()) {