Fix YAMLDecoder's handling when decoding empty strings into optional types

Sources/Yams/Constructor.swift

@@ -396,6 +396,10 @@ extension NSNull/*: ScalarConstructible*/ {
     ///
     /// - returns: An instance of `NSNull`, if one was successfully extracted from the scalar.
     public static func construct(from scalar: Node.Scalar) -> NSNull? {
+        // When constructing from a Scalar, only plain style scalars should be recognized.
+        // For example #"key: 'null'"# or #"key: ''"# should not be considered as null.
+        guard case .plain = scalar.style else { return nil }
+
         switch scalar.string {
         case "", "~", "null", "Null", "NULL":
             return NSNull()

Tests/YamsTests/ConstructorTests.swift

@@ -210,14 +210,16 @@ class ConstructorTests: XCTestCase { // swiftlint:disable:this type_body_length
             english: null
             ~: null key
             ---
-            # This sequence has five
-            # entries, two have values.
+            # This sequence has seven
+            # entries, four have values.
             sparse:
               - ~
               - 2nd entry
               -
               - 4th entry
               - Null
+              - 'null'
+              - ''
 
             """
         let objects = Array(try Yams.load_all(yaml: example))
@@ -234,7 +236,9 @@ class ConstructorTests: XCTestCase { // swiftlint:disable:this type_body_length
                     "2nd entry",
                     NSNull(),
                     "4th entry",
-                    NSNull()
+                    NSNull(),
+                    "null",
+                    ""
                 ]
             ]
         ]

Tests/YamsTests/NodeTests.swift

@@ -59,7 +59,7 @@ class NodeTests: XCTestCase {
         let scalarFloat: Node = "1.0"
         XCTAssertEqual(scalarFloat.float, 1.0)
 
-        let scalarNull: Node = "null"
+        let scalarNull = Node("null", .implicit, .plain)
         XCTAssertEqual(scalarNull.null, NSNull())
 
         let scalarInt: Node = "1"

Tests/YamsTests/TopLevelDecoderTests.swift

@@ -32,5 +32,51 @@ class TopLevelDecoderTests: XCTestCase {
             )
         XCTAssertEqual(foo?.name, "Bird")
     }
+
+    func testDecodeOptionalTypes() throws {
+        let yaml = """
+        AAA: ''
+        BBB:
+        CCC: null
+        DDD: ~
+        EEE: ""
+        json: {
+          "FFF": "",
+          "GGG": "null"
+        }
+        array:
+        - one
+        - ''
+        - null
+        - 'null'
+        - '~'
+        """
+
+        struct Container: Codable, Equatable {
+            struct JSON: Codable, Equatable {
+                var FFF: String?
+                var GGG: String?
+            }
+
+            var AAA: String?
+            var BBB: String?
+            var CCC: Int?
+            var DDD: String?
+            var EEE: String?
+            var json: JSON
+            var array: [String?]
+        }
+
+        let container = try YAMLDecoder().decode(Container.self, from: yaml)
+
+        XCTAssertEqual(container.AAA, "")
+        XCTAssertEqual(container.BBB, nil)
+        XCTAssertEqual(container.CCC, nil)
+        XCTAssertEqual(container.DDD, nil)
+        XCTAssertEqual(container.EEE, "")
+        XCTAssertEqual(container.json.FFF, "")
+        XCTAssertEqual(container.json.GGG, "null")
+        XCTAssertEqual(container.array, ["one", "", nil, "null", "~"])
+    }
 }
 #endif