Empty, unreferenced arrays are not being serialized

[heaths]

When serializing a dictionary, any keys that are empty arrays but unreferenced are not being serialized. Consider the following TOML:

[features]
default = ["foo"]
foo = []
bar = []

A fairly common Cargo.toml manifest with default and non-default features. Deserializing this produces the expected ordered dictionary with array members for all three of default, foo, and bar. Serializing the resulting object only produces:

[features]
default = ["foo"]
foo = []

Notice that bar is missing.

This is causing jborean93/PSToml#10, which we currently use in the Azure SDK for Rust pipelines. If there is a way to work around this for now, that would be great; however, looking through your documentation nothing pops out at me.

[heaths]

With some additional testing of tomlyn directly @hallipr found it's actually just dropping the last entry:

var model = new TomlTable();
model.Add("something", "string value");
model.Add("array1", new TomlArray() { "hello" });
model.Add("array2", new TomlArray() { });
model.Add("array3", new TomlArray() { });
Console.WriteLine(Toml.FromModel(model));

something = "string value"
array1 = ["hello"]
array2 = []

There's no key named "string value" and both array2 and array3 are empty, but only array2 is serialized.

[hallipr]

An even more detailed repro:

var outer = new TomlTable
{
	["inner1"] = new TomlTable {},
	["inner2"] = new TomlTable {
		{ "array1", new TomlArray {} },
		{ "array2", new TomlArray {} }
	},
	["inner3"] = new TomlTable {
		{ "array1", new TomlArray {} },
		{ "array2", new TomlArray {} },
		{ "array3", new TomlArray { "hello" } },
		{ "array4", new TomlArray { } },
		{ "array5", new TomlArray { } },
		{ "array6", new TomlArray { } }
	},
	["inner4"] = new TomlTable {
		{ "array1", new TomlArray {} },
		{ "string", "value" },
		{ "array2", new TomlArray {} },
		{ "array3", new TomlArray {} },
	},
};

Console.WriteLine("---");
Console.WriteLine(Toml.FromModel(outer));
Console.WriteLine("---");

---
[inner1]
[inner3]
array1 = []
array2 = []
array3 = ["hello"]
array4 = []
array5 = []
[inner4]
string = "value"

---

• empty tables are supported and serialize correctly.
• a table containing only empty arrays is not serialized
• a filled array sibling causes empty arrays to serialize except for the last empty array after the filled array
• a string sibling does not cause empty arrays to serialize

[heaths]

To note, my repro was with versoin 0.17.0, which PSToml uses. @hallipr's repro was with 0.18.0, the latest version on nuget.org.