mw-to-git: check blank credential attributes via length

mw-to-git: check blank credential attributes via length

When writing a credential to git-credential, we omit fields
that do not have a true value. This will skip empty or
undefined fields (which we want), but will also accidentally
skip usernames or passwords which happen to have a non-true
value (e.g., "0"). Be more careful by checking for non-zero
length.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>