CMake: Fix how missing backends are handled

- Replace references to the deleted null_backend.py script with an error
  message in CMake.
- Add the missing pw_log_null module.

Change-Id: I7b279cff0c6cd61e97d9c3a622ffd37449319e31
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44600
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>