egl/wayland: add a NULL guard for the authenticate callback

egl/wayland: add a NULL guard for the authenticate callback

On some platforms, the authenticate callback may be NULL, e.g. on
surfaceless. If a client tries to send a wl_drm.authenticate request
the handler tries to dereference the NULL pointer.

This can be reproduced with libva which unconditionally tries to use
wl_drm.authenticate even with render nodes [1]. Run a compositor with
a surfaceless context, then try to start e.g. mpv to trigger the
segfault.

[1]: https://github.com/intel/libva/pull/476

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7992>