Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/benchmarks/semaphore_benchmark.cpp b/benchmarks/semaphore_benchmark.cpp
index a4aa7bb..ba89137 100644
--- a/benchmarks/semaphore_benchmark.cpp
+++ b/benchmarks/semaphore_benchmark.cpp
@@ -75,7 +75,7 @@
   while ((BM_semaphore_sem_post_running > 0) && !sem_wait(semaphore)) {
   }
   BM_semaphore_sem_post_running = -1;
-  return NULL;
+  return nullptr;
 }
 
 class SemaphoreFixture : public benchmark::Fixture {