iothread is a string property, so None (= JSON null) is not a valid
value for it. Pass the empty string instead to get the default iothread.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 tests/qemu-iotests/245 | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/qemu-iotests/245 b/tests/qemu-iotests/245
index bf8261eec0..9b12b42eed 100755
--- a/tests/qemu-iotests/245
+++ b/tests/qemu-iotests/245
@@ -1189,10 +1189,10 @@ class TestBlockdevReopen(iotests.QMPTestCase):
         self.run_test_iothreads('iothread0', 'iothread0')
 
     def test_iothreads_switch_backing(self):
-        self.run_test_iothreads('iothread0', None)
+        self.run_test_iothreads('iothread0', '')
 
     def test_iothreads_switch_overlay(self):
-        self.run_test_iothreads(None, 'iothread0')
+        self.run_test_iothreads('', 'iothread0')
 
 if __name__ == '__main__':
     iotests.activate_logging()
-- 
2.31.1