EnableNullable.cs

using System;
using System.Linq;
using System.Xml.Linq;
using UnityEditor;

public class EnableNullable : AssetPostprocessor
{
    private static string OnGeneratedCSProject(string path, string content)
    {
        var document = XDocument.Parse(content);
        if (document.Root == null) throw new Exception($"document.Root == null");

        XNamespace xNamespace = "http://schemas.microsoft.com/developer/msbuild/2003";

        // ReSharper disable once PossibleNullReferenceException
        var propertyGroup = document
            .Element(xNamespace + "Project")
            .Elements(xNamespace + "PropertyGroup")
            .First();
        propertyGroup.Add(new XElement(xNamespace + "Nullable", "enable"));
        return document.ToString();
    }
}