Fix opening Help page throws Exception
Fix opening Help page throws Exception

- Commit 458c2fa2971e6595a18a289b0afeb4a79ea0e0d3 provoked the error,
  because the "core" "app" was not found. Previously, false was
  returned, but now an AppPathNotFoundException
- IUrlGenerator::linkTo() accepts an empty app argument however, moving
  the "core" portion to the path solves it and avoids apps lookup

Signed-off-by: Arthur Schiwon <9ae9918f3cbf5a90ebc25affe594cc53853aee42@arthur-schiwon.de>

nextcloud/server,nextcloud/server,nextcloud/server,nextcloud/server