apps/ca.c: only output DER with SPKAC input and when -out is chosen

So say the docs

Fixes #8055

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8368)

(cherry picked from commit 69f6b3ceaba493e70e1296880ea6c93e40714f0f)