migration/qmp: Fix crash on setting tls-authz with null

migration/qmp: Fix crash on setting tls-authz with null

QEMU will crash if anyone tries to set tls-authz (which is a type
StrOrNull) with 'null' value.  Fix it in the easy way by converting it to
qstring just like the other two tls parameters.

Cc: qemu-stable@nongnu.org # v4.0+
Fixes: d2f1d29b95 ("migration: add support for a "tls-authz" migration parameter")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20230905162335.235619-2-peterx@redhat.com>
(cherry picked from commit 86dec715a7339fc61c3bdb9715993b277b2089db)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>