Incorrect handling of None in 'in' operator for status field

[megashurik]

Description:
We've discovered an issue with Pony ORM where the 'in' operator doesn't correctly handle None values in certain queries. Specifically, when querying for a status field that can be either an empty string, None, or a specific value, the query fails to return records where the status is None.

Steps to reproduce:

1. Create a database with a table that has a nullable status field.

2. Insert a record with status set to None.

3. Run the following query:

   Tasks.select(lambda t: t.status in ('', None, 'main')).count()

Expected behavior:
The query should return records where status is an empty string, None, or 'main'.

Actual behavior:
The query doesn't return records where status is None.

Workaround:
The issue can be worked around by expanding the condition:

Tasks.select(lambda t: t.status == '' or t.status == None or t.status == 'main').count()

This query correctly returns records with None status.

Environment:

• Pony ORM version: 0.7.19
• Python version: 3.10.11
• Database backend: PostgreSQL
• Operating System: Ubuntu Server 22.04

Additional notes:
This behavior is inconsistent with Python's standard 'in' operator, where None in (None, '', 'main') returns True.