The volume parameter was a more recent addition to the API to support some of the Ads API features. We don’t document the specific value or measure of those fields.
In my case, I’ve just pulled trends via the API for a random location, and I got volumes for a number of them (but not all of them). It’s likely that the null values refer to trending topics that are not at a significant enough threshold to report.
If you wanted to count the Tweets inside a particular trend you’d need to do significant work and probably need to be using our Gnip data products for decent enough fidelity, so my advice would be to use the values you get back, and just omit the volumes where the value is null.
