Use .?method() in more places (#152112)

Switches simple patterns like:

```ts
if (some.thing) {
    some.thing.method();
}
```

to:

```ts
some.thing?.method()
```

This is more concise and avoids having to repeat the `some.thing` part

src/vs/base/browser/defaultWorkerFactory.ts

@@ -86,15 +86,11 @@ class WebWorker implements IWorker {
 	}
 
 	public postMessage(message: any, transfer: Transferable[]): void {
-		if (this.worker) {
-			this.worker.then(w => w.postMessage(message, transfer));
-		}
+		this.worker?.then(w => w.postMessage(message, transfer));
 	}
 
 	public dispose(): void {
-		if (this.worker) {
-			this.worker.then(w => w.terminate());
-		}
+		this.worker?.then(w => w.terminate());
 		this.worker = null;
 	}
 }