make signature optional in SignatureValidationFailed constructor

Author: musou1500

lib/exceptions.ts

@@ -1,7 +1,7 @@
 export class SignatureValidationFailed extends Error {
   constructor(
     message: string,
-    public signature: string,
+    public signature?: string,
   ) {
     super(message);
   }

lib/middleware.ts

@@ -22,7 +22,7 @@ export default function middleware(config: Line.Config & Line.MiddlewareConfig):
     const signature = req.headers["x-line-signature"] as string;
 
     if (!signature) {
-      next(new SignatureValidationFailed("no signature", signature));
+      next(new SignatureValidationFailed("no signature"));
       return;
     }