[FIX] account_financial_report: Remove buggy empty selection value

Selecting no value in the "Grouped by" general ledger wizard, but
filtering any account, results in an empty display, due to the weird
trick of having an empty selection value.

This, joined with the fact of not being able to predefine that empty
value with standard tools, makes the switch necessary.

- Set grouped_by as required with non grouped item description.
- Switched selection value from "" (empty) to "none".
- Adapt general_ledger group_by conditionals.

Co-Authored-By: aritzolea

account_financial_report/wizard/general_ledger_wizard.py

@@ -76,8 +76,9 @@ class GeneralLedgerReportWizard(models.TransientModel):
         help="Ending account in a range",
     )
     grouped_by = fields.Selection(
-        selection=[("", "None"), ("partners", "Partners"), ("taxes", "Taxes")],
+        selection=[("none", "None"), ("partners", "Partners"), ("taxes", "Taxes")],
         default="partners",
+        required=True,
     )
     show_cost_center = fields.Boolean(
         string="Show Analytic Account",