Replace NULL with nullptr or null in webrtc/audio/ and common_audio/.

webrtc/common_audio/vad/include/webrtc_vad.h

Index: webrtc/common_audio/vad/include/webrtc_vad.h
diff --git a/webrtc/common_audio/vad/include/webrtc_vad.h b/webrtc/common_audio/vad/include/webrtc_vad.h
index 91308eef12da8230d388c1f57fe46fbb2b5c9f91..7c11b129643386bb8808d664d80a2a327ec63676 100644
--- a/webrtc/common_audio/vad/include/webrtc_vad.h
+++ b/webrtc/common_audio/vad/include/webrtc_vad.h
@@ -39,7 +39,7 @@ void WebRtcVad_Free(VadInst* handle);
 // - handle [i/o] : Instance that should be initialized.
 //
 // returns        : 0 - (OK),
-//                 -1 - (NULL pointer or Default mode could not be set).
+//                 -1 - (null pointer or Default mode could not be set).
 int WebRtcVad_Init(VadInst* handle);
 
 // Sets the VAD operating mode. A more aggressive (higher mode) VAD is more
@@ -51,7 +51,7 @@ int WebRtcVad_Init(VadInst* handle);
 // - mode   [i]   : Aggressiveness mode (0, 1, 2, or 3).
 //
 // returns        : 0 - (OK),
-//                 -1 - (NULL pointer, mode could not be set or the VAD instance
+//                 -1 - (null pointer, mode could not be set or the VAD instance
 //                       has not been initialized).
 int WebRtcVad_set_mode(VadInst* handle, int mode);