[5.2.x] Fixed #36419 -- Ensured for_save was propagated when resolving expressions.

The for_save flag wasn't properly propagated when resolving expressions, which
prevented get_db_prep_save() from being called in some cases. This affected
fields like JSONField where None would be saved as JSON null instead of SQL NULL.

Regression in 00c690efbc.

Thanks to David Sanders and Simon Charette for reviews.

Co-authored-by: Adam Johnson <me@adamj.eu>

Backport of c1fa3fdd04 from main.

tests/queries/test_bulk_update.py

@@ -2,7 +2,7 @@ import datetime
 
 from django.core.exceptions import FieldDoesNotExist
 from django.db.models import F
-from django.db.models.functions import Lower
+from django.db.models.functions import Coalesce, Lower
 from django.db.utils import IntegrityError
 from django.test import TestCase, override_settings, skipUnlessDBFeature
 
@@ -289,6 +289,21 @@ class BulkUpdateTests(TestCase):
             JSONFieldNullable.objects.filter(json_field__has_key="c"), objs
         )
 
+    @skipUnlessDBFeature("supports_json_field")
+    def test_json_field_sql_null(self):
+        obj = JSONFieldNullable.objects.create(json_field={})
+        test_cases = [
+            ("direct_none_assignment", None),
+            ("expression_none_assignment", Coalesce(None, None)),
+        ]
+        for label, value in test_cases:
+            with self.subTest(case=label):
+                obj.json_field = value
+                JSONFieldNullable.objects.bulk_update([obj], fields=["json_field"])
+                obj.refresh_from_db()
+                sql_null_qs = JSONFieldNullable.objects.filter(json_field__isnull=True)
+                self.assertSequenceEqual(sql_null_qs, [obj])
+
     def test_nullable_fk_after_related_save(self):
         parent = RelatedObject.objects.create()
         child = SingleObject()