audioconvert: Always apply noise when asked

Rename empty.noise -> dither.noise and always add this amount of noise
when > 0. This also adds the noise to silent sounds, not only when
nothing is connected because that would also be a problem when an amp
needs to be kept alive with an non-0 signal.

Rename noise -> dither because we can use this also for dithering later.

See #705

src/daemon/client-rt.conf.in

@@ -90,5 +90,5 @@ stream.properties = {
     #channelmix.rear-delay = 12.0
     #channelmix.stereo-widen = 0.0
     #channelmix.hilbert-taps = 0
-    #empty.noise = 0
+    #dither.noise = 0
 }