Original PR description

Currently, a traceback is occurring when the user tries to sort a website forum posts.

To reproduce this issue:

1) Install website_forum
2) Open `Forum` in the website and tries to sort based on activity

Error:- 
```
AttributeError: 'NoneType' object has no attribute 'groupby'
```

Here in the website_forum post, the query was passed as None, 
and because of the recent changes from the below-mentioned commit, 
a traceback occurs from the line[1].

https://github.com/odoo/odoo/blob/0bac60a7085c51ac8cb4f793b013dbea927d180c/addons/website_forum/controllers/website_forum.py#L126

Line[1]:-
https://github.com/odoo/odoo/blob/0bac60a7085c51ac8cb4f793b013dbea927d180c/odoo/models.py#L5614-L5615

Commit:-
https://github.com/odoo/odoo/pull/183925/commits/6b1df3fddedec486bc20010131ea8da566dade32

We can resolve this issue by passing an empty query instead of None.

sentry-6008918989