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>
t3312-notes-empty: Test that 'git notes' removes empty notes by default

Add test cases documenting the current behavior when trying to
add/append/edit empty notes. This is in preparation for adding
--allow-empty; to allow empty notes to be stored.

Improved-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Johan Herland <johan@herland.net>
notes: Add --allow-empty, to allow storing empty notes

Although the "git notes" man page advertises that we support binary-safe
notes addition (using the -C option), we currently do not support adding
the empty note (i.e. using the empty blob to annotate an object). Instead,
an empty note is always treated as an intent to remove the note
altogether.

Introduce the --allow-empty option to the add/append/edit subcommands,
to explicitly allow an empty note to be stored into the notes tree.

Also update the documentation, and add test cases for the new option.

Reported-by: James H. Fisher <jhf@trifork.com>
Improved-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>