Encode nil values into the document

[marionauta]

The encoder was omitting some nil values, making them undefined in the JavaScript library, instead of null.

Given the following code:

struct Item: Codable {
    var completedAt: Date?

    func encode(to encoder: any Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(completedAt, forKey: .completedAt)
    }
}

let item = Item(completedAt: nil)
let doc = Document()
let encoder = AutomergeEncoder(doc: doc)
try encoder.encode(item)
let completedAt = try doc.get(obj: .ROOT, key: "completedAt")
print(completedAt)

Results in completedAt being nil, instead of ScalarValue.Null. This means the encoder wasn't writing the null value into the Document.

---

This PR changes the encoder so it writes nils into the Document, with tests to check it.

As an implementation detail, now ScalarValue conforms to ScalarValueRepresentable.

[heckj]

Looks great, Mario! Thank you for the detailed tests hitting not just a nil parameter in a map, but an array of potentially nil items as well.