If the opponent's hand is empty, you are unable to initiate Seen in Flames, because it will not change the game state (no cards will go from not looked at to looked at). It is therefore prohibited under "A card ability can only be initiated if its effect has the potential to change the game state" Abilities, p2.
Page 17 of the RRG: "Always resolve a triggering condition before initiating any reactions to that triggering condition." This means you play Seen in Flames, resolve it, and then trigger Melisandre.
- Melisandre can react to a cancelled Seen in Flames. RRG pg 7 "If the effects of an event card are canceled, the card itself is still considered to have been played, and it's costs remain paid."