Fixed #28731 -- Added an error message when using an empty Q() in a When expression.

Otherwise it generates invalid SQL.

tests/expressions_case/tests.py

@@ -1301,3 +1301,8 @@ class CaseWhenTests(SimpleTestCase):
             When(condition=object())
         with self.assertRaisesMessage(TypeError, msg):
             When()
+
+    def test_empty_q_object(self):
+        msg = "An empty Q() can't be used as a When() condition."
+        with self.assertRaisesMessage(ValueError, msg):
+            When(Q(), then=Value(True))