Question: What happens if your opponent has no potential attackers left after Emma’s ability has resolved? Is the Attack Step canceled, or will there still be a Global/Action window?
Answer: Emma’s ability happens before attackers are declared. If your opponent can’t declare an attacker because Emma's ability rerolled them to energy faces, they’ll still have a Global/Action window (since the Attack Step has been initiated). If they have a possible attacker, they must use at least 1 to attack.
The target die is returned to the Field Zone on the rerolled character face.
UPDATE: This ruling is revised based on the more current ruling below:
https://win.wizkids.com/bb/viewtopic.ph ... e8ed43a277
The start of the Attack Step occurs when the Active player declares attackers, whether voluntarily or involuntarily, as in the case of a force to attack ability. Emma’s ability is resolved in the window between the declaring of these attackers and the initiation of any applicable “when attacks” or “when assigned to attack” abilities.
Since attackers have already been declared there is no possibility for the situation stated in the question to occur. The Attack Step will proceed to its conclusion, meaning there will be a global/action window, even if the Active player declared one attacker and it was chosen to be rerolled, resulting in that die being moved to the Reserve Pool (if rolled onto an energy face).