fix(applet-audio): prevent accidental mute on scroll from uninitialized state

[lincolnthalles]

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The issue is easy to replicate:

• Run

  killall cosmic-panel
  

• Hover the volume icon on the panel and scroll down. It will mute the audio regardless of the previous volume level due to an uninitialized state.

Closes #1215

[mmstick]

Conflicts with #1352