fix: a null sub-packet loses its separating space

src/NosCore.Packets/Serializer.cs

@@ -271,9 +271,13 @@ private Expression PacketSerializer(Expression injectedPacket, PacketIndexAttrib
                 incrementExpr = Expression.Constant(!isFromList || !isOptionalSerie);
             }
 
+            // A null sub-packet is written as -1, and it has to carry the same leading separator
+            // the non-null path gets from the discriminator. Without it the -1 is glued to the
+            // field before it and the client cannot split the packet into fields at all:
+            //     sc_n ... 1536-1-1-1 -1 ...   instead of   sc_n ... 1536 -1 -1 -1 ...
             return Expression.Condition(
                 Expression.Equal(specificTypeExpression, Expression.Constant(null, typeof(object))),
-                Expression.Constant(indexAttr.IsOptional ? null : "-1", typeof(object)),
+                Expression.Constant(indexAttr.IsOptional ? null : $"{discriminator}-1", typeof(object)),
                 Expression.Convert(propExp, typeof(object))
             );
         }

test/NosCore.Packets.Tests/SerializerTest.cs

@@ -518,6 +518,26 @@ public void SerializeWithNullFirstParam()
                 packet);
         }
 
+        [TestMethod]
+        public void SerializeWithNullSubPacketKeepsTheSeparator()
+        {
+            var packet = Serializer.Serialize(new ScnPacket
+            {
+                PetId = 1,
+                NpcMonsterVNum = 319,
+                TransportId = 26719,
+                Level = 50,
+                Loyalty = 1000,
+                Experience = 1536,
+                WeaponInstanceDetails = new ScnPacket.ScEquipmentDetails { ItemId = 990 },
+                Name = "Kliff",
+                MorphId = -1
+            });
+
+            Assert.IsTrue(packet.StartsWith("sc_n 1 319 26719 50 1000 1536 990.0.0 -1 -1 -1 "),
+                $"the -1 of a null sub-packet must not be glued to the field before it: {packet}");
+        }
+
         [TestMethod]
         public void SerializeWithSpecialSeparator()
         {