Fixed #35381 -- Added JSONNull() expression.

Thanks Jacob Walls for the review.

tests/model_fields/models.py

@@ -403,6 +403,13 @@ class CustomJSONDecoder(json.JSONDecoder):
         return dct
 
 
+class JSONNullCustomEncoder(json.JSONEncoder):
+    def default(self, o):
+        if isinstance(o, models.JSONNull):
+            return None
+        return super().default(o)
+
+
 class JSONModel(models.Model):
     value = models.JSONField()
 
@@ -422,6 +429,15 @@ class NullableJSONModel(models.Model):
         required_db_features = {"supports_json_field"}
 
 
+class JSONNullDefaultModel(models.Model):
+    value = models.JSONField(
+        db_default=models.JSONNull(), encoder=JSONNullCustomEncoder
+    )
+
+    class Meta:
+        required_db_features = {"supports_json_field"}
+
+
 class RelatedJSONModel(models.Model):
     value = models.JSONField()
     json_model = models.ForeignKey(NullableJSONModel, models.CASCADE)