Add a Converter to serialize empty strings as null values

[MatthewSteeples]

Allows these values to be excluded when IgnoreNullValues is True

[CodeBlanch]

Thanks @MatthewSteeples! I really like the idea, but I'm not sure it completely works. Check out this test:

	JsonSerializerOptions options = new JsonSerializerOptions();
	options.Converters.Add(new JsonEmptyStringToNullConverter());

	string Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One", Str2 = string.Empty, Str3 = null }, options);
	Assert.AreEqual("{\"Str1\":\"One\",\"Str2\":null,\"Str3\":null}", Json);

	options = new JsonSerializerOptions();
	options.Converters.Add(new JsonEmptyStringToNullConverter());
	options.IgnoreNullValues = true;

	Json = JsonSerializer.Serialize(new TestClass() { Str1 = "One", Str2 = string.Empty, Str3 = null }, options);
	Assert.AreEqual("{\"Str1\":\"One\"}", Json); // <-- Fails
	// Actual JSON value: "{\"Str1\":\"One\",\"Str2\":null}"

	private class TestClass
	{
		public string? Str1 { get; set; }
		public string? Str2 { get; set; }
		public string? Str3 { get; set; }
	}

That second test with IgnoreNullValues=true fails. Is the expected behavior that Str2 (empty) & Str3 (null) would both be omitted?