update type to match jwks-rsa

src/index.ts

@@ -6,7 +6,7 @@ import { UnauthorizedError } from './errors/UnauthorizedError';
 /**
  * A function that defines how to retrieve the verification key given the express request and the JWT.
  */
-export type GetVerificationKey = (req: express.Request, token: jwt.Jwt | undefined) => jwt.Secret | Promise<jwt.Secret>;
+export type GetVerificationKey = (req: express.Request, token: jwt.Jwt | undefined) => jwt.Secret | undefined | Promise<jwt.Secret | undefined>;
 
 /**
  * @deprecated use GetVerificationKey