ssl-session-cache: check use on config and availability

ssl-session-cache: check use on config and availability

Replace the check if a ssl session cache is configured with
a function checking if it is configured *and* if an ssl session
cache is available.

During normal operations, a session cache is always there, however
for "connect-only" transfers this might not be the case. When such
transfers receive new sessions/tickets, they need to silently
discard those and not fail.

Reported-by: Marc Aldorasi
Fixes https://github.com/curl/curl/issues/18983
Closes https://github.com/curl/curl/pull/19251