app list: Don't filter out apps with NULL app ID

These are valid when e.g. installing local rpms, or when processing a
org.freedesktop.PackageKit dbus session service request to install a
package that has no appdata.

https://bugzilla.gnome.org/show_bug.cgi?id=777120
https://bugzilla.redhat.com/show_bug.cgi?id=1399152