Depending on the type of session, one or the other might be NULL, which
is not meant to be handled by these functions. Check for both DISPLAY
envvars before setting them on the GAppLaunchContext.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/586
This is a GAppLaunchContext subclass meant to replace usage of
GdkAppLaunchContext in gnome-shell.
Launch contexts get created from the MetaStartupNotification as
they are closely related. The messaging underneath depends on
the availability of a X11 display, if there is one we go through
it (and libsn). If there is none, we still create startup sequences
manually for wayland clients.
