worker: support authentication without X11 display

At the moment we unconditionally set PAM_XDISPLAY
and PAM_XAUTHDATA based on values passed to the worker.

In a future commit, those values are going to become
stubs, so as a first step, this commit makes PAM_XDISPLAY
and PAM_XAUTHDATA optional.

https://bugzilla.gnome.org/show_bug.cgi?id=726283