Is the prayer and fasting of someone valid if they suspected nocturnal emission but found no trace on their clothes, and thought it was madhy, so they performed ablution and prayed?

If you do not find any wetness after waking up, then ghusl (definition) (ritual bath) is not obligatory. If you find wetness and do not know whether it is semen or madhiy (pre-seminal fluid), the preponderant opinion is that ghusl is obligatory. You must repeat every prayer performed before taking ghusl from janabah (definition) (state of ritual impurity), but fasting is valid.