git add --intent-to-add: do not let an empty blob be committed by accident

git add --intent-to-add: do not let an empty blob be committed by accident

Writing a tree out of an index with an "intent to add" entry is blocked.
This implies that you cannot "git commit" from such a state; however you
can still do "git commit -a" or "git commit $that_path".

Signed-off-by: Junio C Hamano <gitster@pobox.com>