[1.4.X] Fixed #17388 - Noted in the custom model field docs that field methods need to handle None if the field may be null.

Backport of 4cef9a09f9 from master

docs/howto/custom-model-fields.txt

@@ -447,6 +447,13 @@ called when it is created, you should be using `The SubfieldBase metaclass`_
 mentioned earlier. Otherwise :meth:`.to_python` won't be called
 automatically.
 
+.. warning::
+
+    If your custom field allows ``null=True``, any field method that takes
+    ``value`` as an argument, like :meth:`~Field.to_python` and
+    :meth:`~Field.get_prep_value`, should handle the case when ``value`` is
+    ``None``.
+
 Converting Python objects to query values
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~