Encode an explicit null for DownloadReference's checksum (#656)

For ease of processing, change the DownloadReference's checksum property to
encode an explicit null into its JSON when no checksum was provided.

rdar://111865266

Author: QuietMisdreavus

Sources/SwiftDocC/Model/Rendering/Tutorial/References/DownloadReference.swift

@@ -101,7 +101,7 @@ public struct DownloadReference: RenderReference, URLReference, Equatable {
         var container = encoder.container(keyedBy: CodingKeys.self)
         try container.encode(type.rawValue, forKey: .type)
         try container.encode(identifier, forKey: .identifier)
-        try container.encodeIfPresent(checksum, forKey: .checksum)
+        try container.encode(checksum, forKey: .checksum)
 
         // Render URL
         if !encodeUrlVerbatim {

Tests/SwiftDocCTests/Rendering/SampleDownloadTests.swift

@@ -277,4 +277,13 @@ class SampleDownloadTests: XCTestCase {
         let downloadReference = try XCTUnwrap(renderNode.projectFiles())
         XCTAssertEqual(downloadReference.url.description, "https://example.com/sample.zip")
     }
+
+    func testExplicitNullChecksum() throws {
+        let identifier = RenderReferenceIdentifier("/test/sample.zip")
+        let originalURL = try XCTUnwrap(URL(string: "/test/sample.zip"))
+        var reference = DownloadReference(identifier: identifier, verbatimURL: originalURL, checksum: nil)
+        let encodedReference = try JSONEncoder().encode(reference)
+        let encodedJson = try XCTUnwrap(String(data: encodedReference, encoding: .utf8))
+        XCTAssert(encodedJson.contains(#""checksum":null"#))
+    }
 }