### Issue:

During migration of Malta taxes, the script can fail when certain tax XML IDs are missing for a company. If the XML ID does not exist, `env.ref(..., raise_if_not_found=False)` returns None. Trying to combine a recordset with None causes the migration to fail. Due to recent [commit]

### Traceback:
```py
    tax_7 |= env.ref(f'account.{company.id}_VAT_S_IN_MT_7_G', raise_if_not_found=False)
  File "/home/odoo/src/odoo/19.0/odoo/orm/models.py", line 6589, in __or__
    return self.union(other)
  File "/home/odoo/src/odoo/19.0/odoo/orm/models.py", line 6603, in union
    raise TypeError(f"unsupported operand types in: {self} | {arg!r}")
TypeError: unsupported operand types in: account.tax() | None
```
### 
Solution:

Use a guard check with the walrus operator (:=) to assign and validate the tax record before union.
This ensures that only existing tax records are added to the recordset, preventing the crash.

Ticket [link1](https://www.odoo.com/odoo/project.task/6159300) [link2](https://www.odoo.com/odoo/project.task/6149387)
opw-6159300 
opw-6149387

Forward-Port-Of: odoo/odoo#264327

Before this commit:
-------------------------
- When planning work orders in the Gantt view, applying a Group By on Work
  Center and then adding another grouping (e.g., Manufacturing Order or
  Status) caused a traceback when clicking grouped rows in the panel.

Steps to reproduce:
-------------------------
1. Install the `mrp_workorder` module.
2. Create and plan a Manufacturing Order.
3. Open the Planning Gantt view
4. Apply Group By → Work Center.
5. Add another group by (e.g., Manufacturing Order or Status)
6. Click on one of the groups (MO or Status) in the side panel.

Cause of the issue:
-------------------------
- Rows grouped by Work Center -> Manufacturing Order attempted to access
  a Work Center record that does not exist, leading to the error:
  `TypeError: Cannot read properties of undefined (reading 'display_name')`
- Rows grouped by Work Center -> Status incorrectly treated the status
  value as a Work Center ID, leading to the error:
  `psycopg2.errors.InvalidTextRepresentation: invalid input syntax for type integer`

After this commit:
-----------------------
- Rows created directly from the Work Center grouping are now correctly
  linked using their corresponding `workcenterIds` and remain clickable.
- Rows created by additional groupings after Work Center (e.g.,
  Manufacturing Order or Status) are no longer clickable.
- This ensures a consistent interaction flow in the Gantt view and prevents the
  previous errors during filtering.

Task ID: 6132396

Forward-Port-Of: odoo/enterprise#114565