AudioCommon/Mixer: Skip sample processing when NullSoundStream is being used.

Source/Core/AudioCommon/NullSoundStream.cpp

@@ -5,6 +5,9 @@
 
 bool NullSound::Init()
 {
+  // Make Mixer aware that audio output is disabled.
+  GetMixer()->SetSampleRate(0);
+
   return true;
 }