[FIX] account: Compute root with single character code

Fixes #47147
Followup on 39e2dfca0a4a2cf641a50ee9af510180fff6eb9c and 737eaf9333

Before this commit, Accounts with single character code was setting non-existing `root_id` which crashes on Grouping by using that field. (and also Accounts were not visible search panel)

With this commit we support computing `root_id` with Single Character.

closes odoo/odoo#48261

X-original-commit: d93ccdc5a25c061468870359ad8f749496140d19
Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>

addons/account/models/account.py

@@ -454,7 +454,7 @@ class AccountAccount(models.Model):
         # This field should have been a char, but the aim is to use it in a side panel view with hierarchy, and it's only supported by many2one fields so far.
         # So instead, we make it a many2one to a psql view with what we need as records.
         for record in self:
-            record.root_id = (ord(record.code[0]) * 1000 + ord(record.code[1:2] or ' ')) if record.code else False
+            record.root_id = (ord(record.code[0]) * 1000 + ord(record.code[1:2] or '\x00')) if record.code else False
 
     @api.depends('code')
     def _compute_account_group(self):