Don't throw an error if there's nothing to show

When the .flatpak directory doesn't exist,
there are no running instances.

Closes: #2027
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f