notmuch: make %N optional for notmuch browser format

It will allow to have vfolder_format option like this:

set vfolder_format="%?N?*& ?%20f %6N %10n"

This format will mark all vfolders with new messages with '*':

                inbox      0          0
*            linux/mm      2        183
     linux/mm/commits      0         39
...

Without the patch all vforlders will be marked.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>