It can be a convenient way to assess a null value in this field for sure. The state of null is usually reached because it was not possible at the time of servicing the request to provide a value in that field – an underlying system could have taken too long to respond, or a null value got cached at some other point in time when that happened.
I guess it would depend on your users – would they prefer a “indeterminate” (or undetermined) state or “zero” value that may be inaccurate?