fix: GREEN - Fixed NPE in `ShardedChatHome.getChatRoom()` for foreign shard

fix: GREEN - Fixed NPE in `ShardedChatHome.getChatRoom()` for foreign shard

* `ShardedChatHome.getChatRoom(UUID)` know checks, if a `ChatHome` exists
  for the selected shard.
* If no `ChatHome` exists, a `ShardNotOwnedException` is thrown.
* The `ChatBackendControllerAdvice` translates the exception to an error
  of type 404 - NOT FOUND, to fullfill the defined expectations.