Fix: useless warning with -snull and no BaseSounds available (#8361)

If I explicitly tell the system I do not want sound, I still get
presented a nice message I do not have any BaseSounds available
on my system, and that I should download one to enjoy sound. Well,
let me tell you, with "-snull" that is really really not going to
help. So please, be quiet, and let me enjoy the game without
"boooooo" and "DING DING DING".

Thank you.

src/sound/null_s.h

@@ -19,6 +19,7 @@ public:
 
 	void Stop() override { }
 	const char *GetName() const override { return "null"; }
+	bool HasOutput() const override { return false; }
 };
 
 /** Factory for the null sound driver. */