Null value not properly serialized

[Thf772]

Description

When serializing, null values (that is, null references, not strings with the value "null") are written as an empty string instead of an actual YAML null. The possibility to serialize null values to actual null should be present either as a default or an option to the SerializerBuilder class.

As a side not, this issue is related to issues #591, #387 and #304 (ambiguous serialization of null-like values between YAML and CLR).

To reproduce

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using YamlDotNet.Serialization;

class Program {
    static async Task Main(string[] args) {
        Dictionary<string, object> data = new() {
            ["null_test"] = null,
        };

        await using MemoryStream outStream = new();
        await using StreamWriter output = new(outStream, new UTF8Encoding());

        ISerializer serializer = new SerializerBuilder().DisableAliases().WithIndentedSequences().Build();
        serializer.Serialize(output, data);

        await output.FlushAsync();
        Console.WriteLine(new UTF8Encoding().GetString(outStream.ToArray()));
    }
}

Expected output:

null_test: null

or

null_test: ~

Actual output:

null_test: 

[yamldotnet]

null_test: null

or

null_test: ~

This YAML snippet appears to be invalid.
The following errors were identifier by the reference parser:

• Line 2, char 1: Unexpected '`'

[pensono]

The empty string is a valid, but not canonical, spelling of the value null.

https://yaml.org/type/null.html

[Thf772]

The empty string is a valid, but not canonical, spelling of the value null.

https://yaml.org/type/null.html

I didn't know empty strings were correct for null values, thanks for clarifying.

Unfortunately, this means that deserializing an object serialized with YamlDotNet will yield a different object, since YamlDotNet itself interprets these null empty strings as non-null empty strings. So there is still a need to offer canonical serialization of nulls...

[EdwardCooke]

This will be fixed when the next release goes out.