Check for X11 session before using X11 paths
Found during development of mutter!420 and https://gitlab.gnome.org/GNOME/mutter/commits/wip/carlosg/xwayland-on-demand. at-spi2-core uses the AT_SPI_BUS root window property as the primary source for the private a11y bus connection if there is a DISPLAY environment variable set.
This is however not sufficient with that mutter work, as there will be a DISPLAY envvar but no Xwayland X server (yet) to back it up. These checks end up spawning Xwayland as a side effect, which would not be expected in a wayland session.
This MR avoids those checks outside a X11 session altogether.