Fixed #35381 -- Added JSONNull() expression.

Thanks Jacob Walls for the review.

docs/topics/db/queries.txt

@@ -1048,13 +1048,15 @@ the following example model::
         def __str__(self):
             return self.name
 
+.. _storing-and-querying-for-none:
+
 Storing and querying for ``None``
 ---------------------------------
 
 As with other fields, storing ``None`` as the field's value will store it as
 SQL ``NULL``. While not recommended, it is possible to store JSON scalar
-``null`` instead of SQL ``NULL`` by using :class:`Value(None, JSONField())
-<django.db.models.Value>`.
+``null`` instead of SQL ``NULL`` by using the :class:`JSONNull()
+<django.db.models.JSONNull>` expression.
 
 Whichever of the values is stored, when retrieved from the database, the Python
 representation of the JSON scalar ``null`` is the same as SQL ``NULL``, i.e.
@@ -1064,18 +1066,21 @@ This only applies to ``None`` as the top-level value of the field. If ``None``
 is inside a :class:`list` or :class:`dict`, it will always be interpreted
 as JSON ``null``.
 
-When querying, ``None`` value will always be interpreted as JSON ``null``. To
-query for SQL ``NULL``, use :lookup:`isnull`:
+When querying, :lookup:`isnull=True <isnull>` is used to match SQL ``NULL``,
+while exact-matching ``JSONNull()`` is used to match JSON ``null``.
+
+.. versionchanged:: 6.1
+
+    ``JSONNull()`` expression was added.
 
 .. code-block:: pycon
 
+    >>> from django.db.models import JSONNull
     >>> Dog.objects.create(name="Max", data=None)  # SQL NULL.
     <Dog: Max>
-    >>> Dog.objects.create(name="Archie", data=Value(None, JSONField()))  # JSON null.
+    >>> Dog.objects.create(name="Archie", data=JSONNull())  # JSON null.
     <Dog: Archie>
-    >>> Dog.objects.filter(data=None)
-    <QuerySet [<Dog: Archie>]>
-    >>> Dog.objects.filter(data=Value(None, JSONField()))
+    >>> Dog.objects.filter(data=JSONNull())
     <QuerySet [<Dog: Archie>]>
     >>> Dog.objects.filter(data__isnull=True)
     <QuerySet [<Dog: Max>]>
@@ -1091,6 +1096,14 @@ Unless you are sure you wish to work with SQL ``NULL`` values, consider setting
     Storing JSON scalar ``null`` does not violate :attr:`null=False
     <django.db.models.Field.null>`.
 
+.. admonition:: Storing JSON ``null`` inside JSON data
+
+    While :class:`JSONNull() <django.db.models.JSONNull>` can be used in
+    :lookup:`jsonfield.key` exact lookups, it cannot be stored inside
+    :class:`dict` or :class:`list` instances meant to be saved in a
+    ``JSONField``, unless a custom encoder is used. If you don't want to use
+    a custom encoder, use ``None`` instead.
+
 .. fieldlookup:: jsonfield.key
 
 .. _key-index-and-path-transforms:
@@ -1122,6 +1135,16 @@ To query based on a given dictionary key, use that key as the lookup name:
     >>> Dog.objects.filter(data__breed="collie")
     <QuerySet [<Dog: Meg>]>
 
+To query a key for JSON ``null``, ``None`` or :class:`JSONNull()
+<django.db.models.JSONNull>` can be used.
+
+.. code-block:: pycon
+
+    >>> Dog.objects.filter(data__owner=None)
+    <Dog: Meg>
+    >>> Dog.objects.filter(data__owner=JSONNull())
+    <Dog: Meg>
+
 Multiple keys can be chained together to form a path lookup:
 
 .. code-block:: pycon