Certainly is very rare! The only quasi positive example I can think of (where its not used as a plot device) is Walt Junior from Breaking Bad. Even though bullies do make fun of him occasionally, it's not a defining characteristic for him, doesn't hold him back, isn't a sign of weakness, is respected by family. More of a "neutral" representation but I always appreciated that they simply let the stutter be a part of his character (as a part of his cerebral palsy) without making a huge deal out of it, or frame it as something that needs to be fixed.