Currently it's possible to mark this widget as indeterminate but it is done by changing other properties in not obvious way (at least it is mentioned in documentation now...).
Instead it would be cleaner and more obvious to have special property to do so, like in QML ProgressBar.
It's also silly that it requires to reset range every time when such state should be toggled which may cause subtle errors.

Getter for such property should check for old approach (range set to 0, 0), but internally separate boolean property could be used to avoid minor data loss (prevent need for range reset).

QStyleOptionProgressBar could make use of explicit toggle too.