scripts: coccicheck: warn on unset debug file

scripts: coccicheck: warn on unset debug file

coccicheck prints debug logs to stdout unless a debug file has been set.
This makes it hard to read coccinelle's suggested changes, especially
for someone new to coccicheck.

From this commit, we warn about this behaviour from within the script on
an unset debug file. Explicitly setting the debug file to /dev/null
suppresses the warning while keeping the default.

Signed-off-by: Benjamin Philip <benjamin.philip495@gmail.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>