allow undefined in ProtoMiddlewareDispatcher

src/middleware-dispatcher/proto-middleware-dispatcher.ts

@@ -8,7 +8,7 @@ export type ProtoMiddlewareArgs<A extends any[] = [], T = any> = [
 ];
 export type ProtoMiddlewareFunc<A extends any[] = [], T = any> = (
   ...args: ProtoMiddlewareArgs<A, T>
-) => T;
+) => T | undefined;
 
 export class ProtoMiddlewareDispatcher<
   A extends any[] = [],