cqlshlib/sslhandling.py: don't use empty userkey/usercert

[fruch]

when configuration doesn't define usercert or userkey

we fail as the following:

Using CQL driver: <module 'cassandra' from '/opt/scylladb/share/cassandra/libexec/../lib/scylla-driver-3.26.3.zip/cassandra/__init__.py'>
Using connect timeout: 5 seconds
Using 'utf-8' encoding
Using ssl: True
Using cloudconf: None
Traceback (most recent call last):
  File "/opt/scylladb/share/cassandra/libexec/cqlsh.py", line 2706, in <module>
    main(*read_options(sys.argv[1:], os.environ))
  File "/opt/scylladb/share/cassandra/libexec/cqlsh.py", line 2647, in main
    shell = Shell(hostname,
            ^^^^^^^^^^^^^^^
  File "/opt/scylladb/share/cassandra/libexec/cqlsh.py", line 490, in __init__
    kwargs['ssl_context'] = sslhandling.ssl_settings(hostname, CONFIG_FILE) if ssl else None
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/scylladb/share/cassandra/libexec/../pylib/cqlshlib/sslhandling.py", line 89, in ssl_settings
    ssl_context.load_cert_chain(certfile=usercert,
TypeError: certfile should be a valid filesystem path

• we don't call load_cert_chain anymore if any of the params are empty
• and now we warn the user if it's missconfigured (if one is and the other isn't)

Fixes: #83

[fruch]

I've test this one with dtest, but I'll need to write a dedicated test for this case (or multiple of them)