Remove deference of null's stub (#1207)

There would be deference of null because stub is null

Author: dumprop

src/protobuf-net/Internal/Serializers/RepeatedDecorator.cs

@@ -10,7 +10,7 @@ internal static class RepeatedDecorator
     {
         public static IRuntimeProtoSerializerNode Create(RepeatedSerializerStub stub, int fieldNumber, SerializerFeatures features, CompatibilityLevel compatibilityLevel, DataFormat dataFormat)
         {
-            if (stub is null) ThrowHelper.ThrowArgumentNullException(nameof(stub), $"No suitable repeated serializer resolved for {stub.ForType.NormalizeName()}");
+            if (stub is null) ThrowHelper.ThrowArgumentNullException(nameof(stub), $"No suitable repeated serializer resolved for stub");
             _ = stub.Serializer; // primes and validates
             return (IRuntimeProtoSerializerNode)Activator.CreateInstance(typeof(RepeatedDecorator<,>).MakeGenericType(stub.ForType, stub.ItemType),
                 new object[] { fieldNumber, features, compatibilityLevel, dataFormat, stub });
@@ -95,4 +95,4 @@ public void EmitWrite(CompilerContext ctx, Local valueFrom)
             ctx.EmitCall(method);
         }
     }
-}
+}