This question already has an answer here:
The following question was proposed by a student durante a lecture for a grad course in EM.
What happens to the energy of an EM wave during destructive interference in vacuum, in regards to the conservation of energy?
Interestingly, for some reason I've never gave the proper thought about it, and while we indeed arrived at a solution, I thought it would be interesting to share it here due to the conceptual importance to undertanding the nature of waves.
I'll post the answer later in the day, so in the meanwhile feel free to offer your answers or interpretations regarding this question (note that there's a similar question on the site but it only deals with material media, so please don't flag this)