tests/qtest/migration: Add a NULL parameters test for TLS

tests/qtest/migration: Add a NULL parameters test for TLS

Make sure the TLS options handling is working correctly with a NULL
parameter. This is relevant due to the usage of StrOrNull for the
tls-creds, tls-authz and tls-hostname options.

With this, all manners of passing TLS options are somehow covered by
the tests, we should not need to do manual testing when touching TLS
options code.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20251215220041.12657-5-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>