[FIX] web: empty a many2one field

Before this commit, it was impossible to empty a many2one in a list view.

How to reproduce:
- go into a list view with a many2one
- empty a many2one containing a value
- click outside the line

Before this commit:
The record goes into readonly mode and its many2one returns to
its original value.

After this commit:
The record is saved and its many2one is empty.

closes odoo/odoo#99140

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>