filter-process: Fix protocol for empty files

filter-process: Fix protocol for empty files

This caused git to complain that filter-process failed and kill it with
signal 15. Because it wrote an extra flushPkt for an empty file, which
git did not expect, and so git saw an unexpected response to the next
request.

Luckily, filter-process is only used by default in v9 and up, and v8 is
still the default. Also, git had to be updating an empty file, followed
by another file, which is a fairly unlikely situation. And git restarts
filter-process after this happens and uses it to filter the rest of the
files. So this isn't a crippling bug.

Sponsored-by: Luke Shumaker on Patreon