do not warn, when requesting the volume from an invalid audio device

Instead, we return a defined value (0.0) and keep quiet otherwise.

src/audio/audio.cpp

@@ -321,13 +321,12 @@ qreal Audio::outputVolume()
 {
     QMutexLocker locker(&d->audioLock);
 
-    if (!d->alMainSource) {
-        qWarning("Audio output source not initialized.");
-        return 0.0;
-    }
+    ALfloat volume = 0.0;
 
-    ALfloat volume;
+    if (d->alOutDev) {
         alGetListenerf(AL_GAIN, &volume);
+        CHECK_AL_ERROR;
+    }
 
     return volume;
 }