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>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9447bda..17ce39b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,7 @@
 add_subdirectory(pw_kvs EXCLUDE_FROM_ALL)
 add_subdirectory(pw_log EXCLUDE_FROM_ALL)
 add_subdirectory(pw_log_basic EXCLUDE_FROM_ALL)
+add_subdirectory(pw_log_null EXCLUDE_FROM_ALL)
 add_subdirectory(pw_log_tokenized EXCLUDE_FROM_ALL)
 add_subdirectory(pw_minimal_cpp_stdlib EXCLUDE_FROM_ALL)
 add_subdirectory(pw_polyfill EXCLUDE_FROM_ALL)