imap-send: fix bug causing cfg->folder being set to NULL

imap-send: fix bug causing cfg->folder being set to NULL

6d1f198f34 (imap-send: fix leaking memory in `imap_server_conf`, 2024-06-07)
resulted a change in static int git_imap_config which resulted in cfg->folder
being incorrectly set to NULL in case imap.user, imap.pass, imap.tunnel and
imap.authmethod were defined. Because of this, since Git 2.46.0,
git-imap-send is not usable at all. The bug seems to have been unnoticed for
a long time, likely due to better options like git-send-email.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>