Fix inconsistent nullability annotation in ReleasableMutexLock

ReleasableMutexLock::Release() resets this pointers, so destructor can compare
it to nullptr, to avoid double release.
PiperOrigin-RevId: 803163168
Change-Id: Ib97f86a4e5fca789552b1a8c385868d4227b9874

absl/synchronization/mutex.h

diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index 77ab7c4..dce8b8f 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -1123,7 +1123,7 @@
   void Release() ABSL_UNLOCK_FUNCTION();
 
  private:
-  Mutex* absl_nonnull mu_;
+  Mutex* absl_nullable mu_;
   ReleasableMutexLock(const ReleasableMutexLock&) = delete;
   ReleasableMutexLock(ReleasableMutexLock&&) = delete;
   ReleasableMutexLock& operator=(const ReleasableMutexLock&) = delete;