Are my puberty and wet dream valid if I dreamt that I was masturbating and semen was emitted in the dream, but I found no trace of it on my body or clothes after waking up? And does God forgive one who committed Shirk while knowing, then repented and sought forgiveness?

The signs of puberty are what determine legal accountability (taklif shar'i), not dreams, as long as semen has not been discharged. As for repentance, it erases what came before it, and Allah accepts the repentance of a polytheist and an apostate as long as the sun has not risen from the west or the soul has not reached the throat. This is in accordance with His, the Almighty’s, saying: "Say to those who have disbelieved, if they cease, what has passed will be forgiven for them," and His, the Almighty’s, saying: "Except for those who repent, believe, and do righteous deeds; for them Allah will change their evil deeds into good ones," and the saying of the Prophet, peace be upon him: "Indeed, Allah accepts the repentance of a servant as long as he has not gurgled [i.e., is not on his deathbed]."