submodule--helper: don't print null in 'submodule status'

The function compute_rev_name() can return NULL sometimes (e.g. right after 'submodule init'). The current code makes 'submodule status' print this: 19d97bf5af05312267c2e874ee6bcf584d9e9681 sha1collisiondetection ((null)) This ugly 'null' adds no value to the user using this command. More importantly printf() on some platform can't handle NULL as a string and will crash instead of printing '(null)'. Check for this and skip printing this part (the alternative is printing '(n/a)' or something but I think that is just noise). Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Reviewed-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>