[FIX] mail: Check field exists when computing groups

Steps to reproduce:

1. install `hr_contract_salary`

2. create a contract

3. set or update `hr_responsible_id` on the contract
(it creates a `mail.tracking.value`)

4. uninstall `hr_contract_salary`

5. go to the contract form view
=> traceback `hr_responsible_id` does not exist

Before displaying a `mail.tracking.value`, `groups` of the
fields are checked to fitler tracking values according to access
rights of the user.
However if the module that added the field has been uninstalled,
the field does not exists anymore.

Fix:
check if field exists before getting fields groups.
If the field does not exists: set groups to `base.group_system`

closes odoo/odoo#31398

Signed-off-by: "Yannick Tivisse (yti)" <yti@odoo.com>