cli/show: avoid empty write to stdout in format_part_raw

cli/show: avoid empty write to stdout in format_part_raw

Previously if the input was exactly a multiple of the internal buffer
size, notmuch would attempt to fwrite nothing to stdout, but still
expected fwrite to return 1, causing a failure that looked like this:

  $ notmuch show --format=raw id:87o96f1cya.fsf@codeaurora.org
    ...entire message shown as expected..
  Error: Write failed
  $ echo $?
  1

To fix the problem don't call fwrite at all when there's nothing to
write.

Amended by db: add some tests of message sizes likely to cause this
problem.