#648 may have been a bit hasty - I realised afterward that there's a
simpler way to achieve the same thing, and include the Brute filter as
well.
This reverts #648 and instead just picks None up front if the line is
all zeros. This is guaranteed to be the chosen filter for MinSum,
Entropy, Bigrams and BigEnt. It's almost certainly true for Brute as
well but this is harder to prove. I've tested this across hundreds of
images and found no change in output.
ca05d99be4