Adjust PQsslAttributeNames() to match PQsslAttribute().

Adjust PQsslAttributeNames() to match PQsslAttribute().

Currently, PQsslAttributeNames() returns the same list of attribute
names regardless of its conn parameter.  This patch changes it to
have behavior parallel to what 80a05679d installed for PQsslAttribute:
you get OpenSSL's attributes if conn is NULL or is an SSL-encrypted
connection, or an empty list if conn is a non-encrypted connection.
The point of this is to have sensible connection-dependent behavior
in case we ever support multiple SSL libraries.  The behavior for
NULL can be defined as "the attributes for the default SSL library",
parallel to what PQsslAttribute(NULL, "library") does.

Since this is mostly just future-proofing, no back-patch.

Discussion: https://postgr.es/m/17625-fc47c78b7d71b534@postgresql.org