Fixing ICacheObject::get return type (#29001)

get can return undefined if the key isn't in the cache.

See the example:
https://docs.angularjs.org/api/ng/type/$cacheFactory.Cache
```
  superCache.remove('another key');
  expect(superCache.get('another key')).toBeUndefined();
```

types/angular/index.d.ts

@@ -1316,7 +1316,7 @@ declare namespace angular {
          *
          * @param key the key of the data to be retrieved
          */
-        get<T>(key: string): T;
+        get<T>(key: string): T | undefined;
 
         /**
          * Removes an entry from the Cache object.