builtin/notes: fix premature failure when trying to add the empty blob

builtin/notes: fix premature failure when trying to add the empty blob

This fixes a small buglet when trying to explicitly add the empty blob
as a note object using the -c or -C option to git notes add/append.
Instead of failing with a nonsensical error message indicating that the
empty blob does not exist, we should rather behave as if an empty notes
message was given (e.g. using -m "" or -F /dev/null).

The next patch contains a test that verifies the fixed behavior.

Found-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>