Visit Duration is not dependent on the number of hits or page views, rather it is dependent on the time between hits.. Your question cant be answered In my opinion with knowing how many page views or hits you are getting. If you are getting crazy traffic then your 0 visit duration could be accurate, if your page views or hits is low then yes it could be robots...

Duration in GA - is the average time duration of the visits. If you are buying traffic this is typical. If your traffic is organic in nature then its something deeper and could be an issue with your site.

So take a look at your traffic sources and see which ones are driving the duration on site lower and which ones are higher. The lower ones are obviously not in your target market if they are coming to your site and then leaving immediately.