In the case where both vectors are all zeros, the angle between them is 0, so the cosine is therefore 1 and the so the distance returned should be 0 (unless I misunderstood what the distance does).
In Mahout, when calling distance() however, if both the denominator and dotProduct are 0 (which is true when both vectors are 0), the returned value is 1. This looks like a bug to me and I would open a JIRA issue and fix it but I want to make sure there's nothing I could possibly be missing. Thoughts?
