Make ca command silently use default if .attr file does not exist

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7286)

(cherry picked from commit ac454d8d4663e2fcf8a8437fab8aefd883091c37)