parental-controls: fix read of uninitialized variable
parental-controls: fix read of uninitialized variable

If content_rating == NULL, then no value will be assigned to
appdata_value, but its value will be used anyway – if it happens to be
non-NULL, it will be dereferenced.

    common/flatpak-parental-controls.c: In function ‘flatpak_oars_check_rating’:
    common/flatpak-parental-controls.c:121:10: warning: ‘appdata_value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
           if (appdata_value != NULL)
              ^

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