Is ghusl obligatory for someone who had a wet dream but found no trace of semen, then after waking up, a thin, viscous fluid followed?

It is not obligatory to perform ghusl if semen is not found after a wet dream, and what is found after waking up and urinating does not necessitate ghusl, because it is not semen; rather, it is perhaps prostatic fluid (wadi). Your action is correct and your prayer is sufficient.