Nullptr check for VSyncClient::Wait

as the documentation in the header implies

Bug: 34609900
Test: pass nullptr to Wait and observe your code not crashing
Change-Id: Ia3ead8cf3369bddedfca4606d0fa261fd57c281f
(cherry picked from commit a919630f3c7cd3c2ee90ef39cbf8147681dd9be8)
diff --git a/libs/vr/libdisplay/vsync_client.cpp b/libs/vr/libdisplay/vsync_client.cpp
index a0fb313..c928a08 100644
--- a/libs/vr/libdisplay/vsync_client.cpp
+++ b/libs/vr/libdisplay/vsync_client.cpp
@@ -26,7 +26,10 @@
           status.GetErrorMessage().c_str());
     return -status.error();
   }
-  *timestamp_ns = status.get();
+
+  if (timestamp_ns != nullptr) {
+    *timestamp_ns = status.get();
+  }
   return 0;
 }