refactor(cdk/a11y): only call currentResolve if defined (#28701)

Should it be possible for it to be undefined within the timeout body?
No. But ZoneJS and test environments do weird things so it's better to
be safe.

(cherry picked from commit b13818a)

Author: atscott

src/cdk/a11y/live-announcer/live-announcer.ts

@@ -135,7 +135,9 @@ export class LiveAnnouncer implements OnDestroy {
           this._previousTimeout = setTimeout(() => this.clear(), duration);
         }
 
-        this._currentResolve!();
+        // For some reason in tests this can be undefined
+        // Probably related to ZoneJS and every other thing that patches browser APIs in tests
+        this._currentResolve?.();
         this._currentPromise = this._currentResolve = undefined;
       }, 100);