hashpumpy: do not need to check if original_data_size == 0

[gengkev]

The hashpumpy.hashpump function requires that the original_data argument be nonempty. But this is unnecessary; an empty string for original_data works fine as well, and should emit at most a warning, but not an error. I fixed this for my own purposes by removing the if-statement starting at https://github.com/bwall/HashPump/blob/master/hashpumpy.cpp#L64.