[NTG-context] numberstopper in \setupcaptions

Hello list members, I am not sure how to achieve the following functionality: Using \startplacefigure, I would like to place a number stopper *only* when the caption text is non-empty, i.e. Figure 1 Figure 2: A cow This would be achieved through \setupcaption [figure] using numberstopper= I believe (with perhaps a filter), but I am not sure of exactly how to do this, especially how to do this most elegantly. Thank you Alan