pw_log_null: Missing public dependency and target

- Add missing public dependency dir_pw_preprocessor
- Add required target pw_log_null.impl

Change-Id: Id7302a350fa35d5941c28cdc6520ea555239da6a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48300
Reviewed-by: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>

pw_log_null/BUILD.gn

diff --git a/pw_log_null/BUILD.gn b/pw_log_null/BUILD.gn
index f4b546a..ff5ddcb 100644
--- a/pw_log_null/BUILD.gn
+++ b/pw_log_null/BUILD.gn
@@ -31,6 +31,11 @@
   public = [ "public_overrides/pw_log_backend/log_backend.h" ]
   sources = [ "public/pw_log_null/log_null.h" ]
   friend = [ ":*" ]
+  public_deps = [ dir_pw_preprocessor ]
+}
+
+pw_source_set("pw_log_null.impl") {
+  sources = []
 }
 
 pw_doc_group("docs") {