Filtering out multiple string values as null values when deserializing a JSON string

Program.cs

using System.Text.Json;
using System.Text.Json.Nodes;
using System.Text.Json.Serialization;

namespace JsonConvertExample;

internal static class Program
{
    private static void Main()
    {
        const string inputJson = """{ "prop1":"val", "prop2":10, "prop3":"", "prop4":["val", "-", "val2"], "prop5":{ "sub1":"val", "sub2":"N/A" } }""";

        var options = new JsonSerializerOptions
        {
            WriteIndented = false,
            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
        };
        
        options.Converters.Add(new CustomJsonConverter());

        var document = JsonSerializer.Deserialize<JsonNode>(inputJson, options);
        Console.WriteLine(JsonSerializer.Serialize(document, options));
    }
}

public class CustomJsonConverter : JsonConverter<JsonNode>
{
    public override JsonNode? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var node = JsonNode.Parse(ref reader);
        return FilterJsonNode(node);
    }

    private static bool IsNullValue(string value)
    {
        return value.ToUpper() is "" or "-" or "N/A";
    }

    private static JsonNode? FilterJsonNode(JsonNode? node)
    {
        switch (node)
        {
            case JsonObject jsonObject:
            {
                foreach (var (key, value) in jsonObject.ToList())
                {
                    if (value is JsonValue jsonValue)
                    {
                        if (jsonValue.TryGetValue(out string? stringValue)
                            && IsNullValue(stringValue))
                        {
                            jsonObject.Remove(key);
                        }
                    }
                    else
                    {
                        var filteredValue = FilterJsonNode(value);
                        if (filteredValue is not null)
                        {
                            jsonObject[key] = filteredValue;
                        }
                        else
                        {
                            jsonObject.Remove(key);
                        }
                    }
                }

                break;
            }
            case JsonArray jsonArray:
            {
                for (var i = 0; i < jsonArray.Count; i++)
                {
                    var value = jsonArray[i];
                    if (value is JsonValue jsonValue
                        && jsonValue.TryGetValue(out string? stringValue)
                        && IsNullValue(stringValue))
                    {
                        jsonArray.RemoveAt(i);
                        i--;
                    }
                    else
                    {
                        var filteredValue = FilterJsonNode(value);
                        if (filteredValue is not null)
                        {
                            jsonArray[i] = filteredValue.DeepClone();
                        }
                        else
                        {
                            jsonArray.RemoveAt(i);
                            i--;
                        }
                    }
                }

                break;
            }
        }

        return node;
    }

    public override void Write(Utf8JsonWriter writer, JsonNode value, JsonSerializerOptions options)
    {
        value.WriteTo(writer);
    }
}