[FIX] base: fix ir_attachment read_group

When called with a `srt` as readgroup parameter we get a traceback.
Example:
`read_group([('partner_id', 'in', self.ids)], 'partner_id', 'partner_id')`
TB:
```
 Traceback (most recent call last):
   File "/home/odoo/src/odoo/14.0/odoo/tools/safe_eval.py", line 330, in safe_eval
    return unsafe_eval(c, globals_dict, locals_dict)
   File "", line 2, in <module>
   File "/home/odoo/src/odoo/14.0/odoo/addons/base/models/ir_attachment.py", line 420, in read_group
    if any('(' in field for field in fields + groupby):
 TypeError: can only concatenate list (not "str") to list
 ```

 Observed on the upgrade request 22627.

closes odoo/odoo#75410

X-original-commit: 8e7990dd5c069f7b0dba9f3d27f620f0fcad5441
Signed-off-by: Christophe Simonis <chs@odoo.com>