TVB-2758. Ensure values of zero work for float fields

Created by: popaula937

On the base Field class, we have a value property that does not work correctly when float fields are zero. It ended up in returning a string '0' in the view_stimulus endpoint, thus the error. I added an explicit check for this situation, as it is the quickest solution. But I believe this code needs a larger review. Maybe should be done together with the task TVB-2669, where we will also clean up the unnecessary Field classes.