applet: Fix incompatible null binding to a string property

Fixes this warning:

    file:///usr/share/plasma/plasmoids/org.kde.plasma.volume/contents/ui/main.qml:603:39: Invalid property assignment: string expected - Assigning null to incompatible properties in QML is deprecated. This will become a compile error in future versions of Qt.

Amends 0e8d4688.