I think that given the way the death of Aslan closely paralleled the story of the Passion, it is not a significant omission to exclude one detail which doesn't come into play in the story and would only cloud the story further.
I am not a Christian, but I enjoyed the movie for what it was and am happy that Christian parents will have this splendid tool for their children to learn the story of Christ in a different form. I don't think they were trying to white out God or anything but not mentioning the Emperor, I just think it's not relevant to the story and certainly not needed to get the point across. I mean, come on, Susan and Lucy hugging Aslan in the garden before he makes his sacrifice and then going to collect his body after he's killed? You can't accuse them of cutting corners on the allegory.