Raise minimum Yams version.

Fixes #364

Author: brandonbloom

Package.resolved

@@ -21,7 +21,7 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
-        .package(url: "https://github.com/jpsim/Yams.git", "4.0.0"..<"6.0.0") // just for tests
+        .package(url: "https://github.com/jpsim/Yams.git", "5.1.0"..<"6.0.0") // just for tests
     ],
     targets: [
         .target(

Package.swift

@@ -16,6 +16,20 @@ import OpenAPIKit
 import Yams
 
 final class SchemaObjectYamsTests: XCTestCase {
+    func test_nullTypeDecode() throws {
+        let nullString =
+        """
+        type: 'null'
+        """
+
+        let null = try YAMLDecoder().decode(JSONSchema.self, from: nullString)
+
+        XCTAssertEqual(
+            null,
+            JSONSchema.null()
+        )
+    }
+
     func test_floatingPointWholeNumberIntegerDecode() throws {
         let integerString =
         """