[FIX] database_cleanup: filter on many2many fields

Following odoo/odoo@54238c88e85d6e a Many2many may have no relation
This avoids having a None record in the list (which is problematic with the
NOT IN clause)
https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_NOT_IN

database_cleanup/models/purge_tables.py

@@ -93,6 +93,7 @@ class CleanupPurgeWizardTable(models.TransientModel):
                 for column in model_pool._fields.values()
                 if column.type == 'many2many' and
                 (column.compute is None or column.store)
+                and column.relation
             ]
 
         self.env.cr.execute(