Is there a difference between a "contribution" and a "positive contribution"?

Yes, there is a difference.

The word "positive" has many meanings.

In the context of the phrase "positive contribution", the most likely meaning is the one given as 3 by the American Heritage Dictionary:

a. Desirable, admirable, or beneficial: a woman with many positive qualities; the positive features of this new software.
b. Optimistic or constructive: a positive attitude.
c. Not disparaging or malicious: ran a positive political campaign.

If you say something like "I don't think that Bailey made any positive contributions to the conversation", it's implied that even if Bailey took part in the conversation (and thus could be said to have "contributed" to it), the contributions were not constructive or not beneficial.