Fix bug in wait_for_confident_result

[CoreyEWood]

The confidence_threshold parameter defaults to None, and the docstring states that if it is None then the detector's confidence threshold will be used. However, it currently only fetches the detector's confidence threshold if the value for image_query is a string - if you pass it an ImageQuery object, confidence_threshold will still be None when passed to iq_is_confident, which will then error when trying to do iq.result.confidence >= confidence_threshold.

This fixes it so it will always fetch the detector's confidence threshold if the inputted confidence_threshold is None.

[brandon-wada]

Nice catch