Fixed #36453 -- Made When.condition resolve with for_save=False.

Value(None, JSONField()) when used in When.condition incorrectly resolved with
for_save=True, resulting in the value being serialized as SQL NULL instead of
JSON null.

Regression in c1fa3fdd04.

Thanks to Thomas McKay for the report, and to David Sanders and Simon Charettes
for the review.

Co-authored-by: Sarah Boyce <42296566+sarahboyce@users.noreply.github.com>

tests/expressions/tests.py

@@ -29,6 +29,7 @@ from django.db.models import (
     FloatField,
     Func,
     IntegerField,
+    JSONField,
     Max,
     Min,
     Model,
@@ -83,6 +84,7 @@ from .models import (
     Company,
     Employee,
     Experiment,
+    JSONFieldModel,
     Manager,
     Number,
     RemoteEmployee,
@@ -367,6 +369,21 @@ class BasicExpressionsTests(TestCase):
             Number.objects.all(), [None, None], lambda n: n.float, ordered=False
         )
 
+    @skipUnlessDBFeature("supports_json_field")
+    def test_update_jsonfield_case_when_key_is_null(self):
+        obj = JSONFieldModel.objects.create(data={"key": None})
+        updated = JSONFieldModel.objects.update(
+            data=Case(
+                When(
+                    data__key=Value(None, JSONField()),
+                    then=Value({"key": "something"}, JSONField()),
+                ),
+            )
+        )
+        self.assertEqual(updated, 1)
+        obj.refresh_from_db()
+        self.assertEqual(obj.data, {"key": "something"})
+
     def test_filter_with_join(self):
         # F Expressions can also span joins
         Company.objects.update(point_of_contact=F("ceo"))