Empty quoted strings are parsed as null

[JensAyton]

Note: This is not the same as #253. However, the issue @tjprescott referenced in #253 is actually this.

---

When decoding a document like this:

stringA: "Hello"
stringB: ""

into a struct with optional string members like this:

struct TestType: Codable {
    var stringA: String?
    var stringB: String?
    var stringC: String?
}

stringB is read as nil rather than an empty string. If TestType.stringB is made non-optional, it’s read as an empty string.

On further inspection, this could also happen for string values of "~", "null", "Null" or "NULL". This is surprising and dangerous behaviour.

The YAML spec explicitly states: “Note that a null is different from an empty string.” While the style of the spec is confusing, I don’t believe it intends for the string "null" to be of null type either.

This can be fixed by changing NSNull.construct to return nil unless scalar.style == .plain. However, I’m leery of making a PR with this change since it may impact existing code with unfortunate dependencies on the current behaviour.

[JensAyton]

Full example:

import Foundation
import Yams

let testYAML = #"""
stringA: "Hello"
stringB: ""
"""#

let testJSON = #"""
{
    "stringA": "Hello",
    "stringB": ""
}
"""#.data(using: .utf8)!

struct TestType: Codable {
    var stringA: String?
    var stringB: String?
    var stringC: String?
}

// Prints wrong result TestType(stringA: Optional("Hello"), stringB: nil, stringC: nil)
let testFromYAML = try! YAMLDecoder().decode(TestType.self, from: testYAML)
print(String(reflecting: testFromYAML))

// Prints correct result TestType(stringA: Optional("Hello"), stringB: nil, stringC: nil)
let testFromJSON = try! JSONDecoder().decode(TestType.self, from: testJSON)
print(String(reflecting: testFromJSON))

[jpsim]

Fixed in 0c4ff78.