BUG/MINOR: ssl: Fix potential NULL-deref in "dump ssl cert" with AWS-LC

BUG/MINOR: ssl: Fix potential NULL-deref in "dump ssl cert" with AWS-LC

In some corner cases we could have a public certificate with no private
key stored in a crt-store. In such a case, when trying to dump the
contents of the store entry via the "dump ssl cert" CLI command, the
'PEM_write_bio_PrivateKey' was always called without checking for a NULL
pointer.
This poses no problems with OpenSSL libraries which already check the
pointer but on some other libs, AWS-LC for instance, the NULL pointer is
dereferenced without check.
The only other use of this function is in acme code where the function
can never be called with a NULL 'key' pointer.

This patch can be backported up to 3.2.