app: Properly return NULL not FALSE
app: Properly return NULL not FALSE

This probably doesn't make any difference but at least the code is more
readable.

Closes: #2593
Approved by: matthiasclasen

matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak