bpo-40904: Fix segfault in the new parser with f-string containing yield statements with no value (GH-20701)

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index ea4e589..9048e89 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -725,9 +725,11 @@
         #  a function into a generator
         def fn(y):
             f'y:{yield y*2}'
+            f'{yield}'
 
         g = fn(4)
         self.assertEqual(next(g), 8)
+        self.assertEqual(next(g), None)
 
     def test_yield_send(self):
         def fn(x):