Fix nullable struct enum deserialization

src/JsonRpc/Serialization/Converters/EnumLikeStringConverter.cs

@@ -8,16 +8,34 @@ internal class EnumLikeStringConverter : JsonConverter<IEnumLikeString>
     {
         public override void WriteJson(JsonWriter writer, IEnumLikeString value, JsonSerializer serializer) => new JValue(value.ToString()).WriteTo(writer);
 
-        public override IEnumLikeString ReadJson(
+        public override IEnumLikeString? ReadJson(
             JsonReader reader, Type objectType, IEnumLikeString existingValue,
             bool hasExistingValue,
             JsonSerializer serializer
         ) =>
             reader.TokenType switch {
-                JsonToken.String => (IEnumLikeString) Activator.CreateInstance(objectType, (string) reader.Value),
+                JsonToken.String => (IEnumLikeString?) CreateEnumLikeString(objectType, (string?) reader.Value),
                 _                => (IEnumLikeString) Activator.CreateInstance(objectType, null)
             };
 
         public override bool CanRead => true;
+
+        private static object? CreateEnumLikeString(Type objectType, string? value)
+        {
+            if (objectType.IsGenericType
+                && objectType.GetGenericTypeDefinition() == typeof(Nullable<>))
+            {
+                // TODO: If the value is null, should we return null? ReadJson()'s return type isn't nullable...
+                if (value is null)
+                {
+                    return null;
+                }
+
+                // We could also create a Nullable<T>, but then will that be an IEnumLikeString?
+                return Activator.CreateInstance(objectType.GetGenericArguments()[0], value);
+            }
+
+            return Activator.CreateInstance(objectType, value);
+        }
     }
 }