Make sure we have IDs and display names

Return non-NULL strings from flatpak_dir_get_id()
and flatpak_dir_get_display_name() for user installations,
to save library users the hassle of dealing with NULL
return values.

Closes: #2583
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f