
There are cases where this._user might be null when a session
is opened. This is because the user is not selected via GDM but
it is set through PAM. This happens when logging with smart card
or with credential managers, for example.
Given the session-id of the opened session, look for the owner.
This fixes the following error:
```
gnome-shell[153293]: TypeError: this._user is null
Stack trace:
_findConflictingSession@resource:///org/gnome/shell/gdm/loginDialog.js:1219:26
_onSessionOpened@resource:///org/gnome/shell/gdm/loginDialog.js:1254:51
@resource:///org/gnome/shell/ui/init.js:21:20
```
Fixes: df84854d9
("loginDialog: On login, allow logout a conflicting session")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7526
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3448>