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>