make listener callback on patcher subscribe optional

src/main/services/patcher/patcher.ts

@@ -145,7 +145,7 @@ export class Patcher<T> {
    * @param listener A function that will be called when the object changes.
    * @returns A subscription ID that can be used to unsubscribe.
    */
-  subscribe(listener: PatchListener): number {
+  subscribe(listener?: PatchListener): number {
     const key = this.nextKey();
     this.subscribers[key] = this.observable.subscribe(listener);
 

src/tests/unit/services/patcher/patcher-test.ts

@@ -154,7 +154,7 @@ describe('patcher class.', () => {
   test('suppresses patches on a changed address for a limited time.', async () => {
     const patcher = new Patcher();
     patcher.initialize({ x: 42 });
-    patcher.subscribe(() => {});
+    patcher.subscribe();
     patcher.check({ x: 43 });
 
     await sleep(1);