Callback of origin should be potentially undefined

In reference to fastify#236 origin cannot be assumed to always be a string. Propose making it `string | undefined` to ensure people handle it correctly. This would flag quickly in the sync example on the README that new URL can't work with undefined.

index.d.ts

@@ -22,7 +22,7 @@ type FastifyCorsHook =
   | 'onSend'
 
 declare namespace fastifyCors {
-  export type OriginFunction = (origin: string, callback: OriginCallback) => void;
+  export type OriginFunction = (origin?: string, callback: OriginCallback) => void;
 
   export interface FastifyCorsOptions {
     /**