fix compile warning, which will be compile error in upcoming swift 6

Author: luoxiu

Source/SwiftyJSON/SwiftyJSON.swift

@@ -236,7 +236,7 @@ public struct JSON {
                 rawString = string
             case _ as NSNull:
                 type = .null
-            case nil:
+            case Optional<Any>.none:
                 type = .null
             case let array as [Any]:
                 type = .array

Tests/SwiftJSONTests/RawRepresentableTests.swift

@@ -84,6 +84,12 @@ class RawRepresentableTests: XCTestCase {
         if JSON(rawValue: NSObject()) != nil {
             XCTFail("Should not run into here")
         }
+        
+        do {
+            let n: Int? = nil
+            let json = JSON(n as Any)
+            XCTAssertEqual(json.type, .null)
+        }
     }
 
     func testArray() {