Fixing ICacheObject::get return type (#29001)
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();
```
georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,magny/DefinitelyTyped,AgentME/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,markogresak/DefinitelyTyped,magny/DefinitelyTyped,mcliment/DefinitelyTyped