Merge branch 'jh/filter-empty-contents' into maint

The clean/smudge interface did not work well when filtering an
empty contents (failed and then passed the empty input through).
It can be argued that a filter that produces anything but empty for
an empty input is nonsense, but if the user wants to do strange
things, then why not?

* jh/filter-empty-contents:
  sha1_file: pass empty buffer to index empty file

diff --cc sha1_file.c
Simple merge

diff --cc t/t0021-conversion.sh
Simple merge