Inversed logic has been proven to be more difficult to read,
so use the simple positive variant.
Also add a simple sanity check for `WAYLAND_DISPLAY` so if people
set `MOZ_ENABLE_WAYLAND` in a X11 session don't get undesired behavior.
While on it, change a check for `XDG_SESSION_TYPE` to also use
`WAYLAND_DISPLAY`, improving behavior when launching FF from a TTY
or a TTY-launched session (e.g. via `weston-launch`).
`WAYLAND_DISPLAY` and `DISPLAY` are not expected to be set if
no Wayland or X11 server is available, so using them makes us behave
more predictable.
Differential Revision: https://phabricator.services.mozilla.com/D106726
f1206ef5d7