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

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>