Problem: incorrect internal diff with an empty file
Solution: Set pointer to NULL, instead of using an empty line file
(Yukihiro Nakadaira)
When using internal diff, empty file is read as one empty line file.
So result differs from external diff.
closes: vim/vim#15719f1694b439b
Co-authored-by: Yukihiro Nakadaira <yukihiro.nakadaira@gmail.com>