permission-db: Do not crash on NULL child_app_id

[A6GibKm]

The callers of this function check whether the return value is NULL.

Fixes: #1616

[swick]

The change itself LGTM, but the underlying problem remains: somehow a NULL app_id managed to get into your permission db and that should not happen. Not sure if this really should close the reported issue.

[A6GibKm]

$ flatpak permission-show ca.desrt.dconf-editor
Table         Object       App                   Permissions Data
notifications notification ca.desrt.dconf-editor yes         0x00

$ flatpak permissions | awk '{ print $3 }'
org.mozilla.Thunderbird
org.gnome.Calculator
org.gnome.Fractal
org.gnome.World.PikaBackup
org.gnome.clocks
ca.desrt.dconf-editor
com.github.marhkb.Pods
com.spotify.Client
im.riot.Riot
org.gnome.Builder
org.gnome.Fractal
org.gnome.World.PikaBackup
org.gnome.baobab
org.gnome.clocks
org.mozilla.Thunderbird
org.mozilla.firefox
org.signal.Signal
org.gnome.Calendar
org.gnome.Maps
org.gnome.Weather
org.mozilla.firefox
com.belmoussaoui.ashpd.demo
org.mozilla.firefox
org.mozilla.firefox
org.mozilla.firefox
org.mozilla.firefox
org.mozilla.firefox
org.mozilla.firefox

Is there something I am missing? It does not look to have any NULL app id. Is there any way manually inspect the permissions on this machine?