[FIX] account: aged partner: partner can be unknown

Have one move with
- one debit in receivable with a partner which has a partner.category on it
- one other debit in receivable, without a partner
- one credit wherever

Open the aged partner balance report, filter partners on the relevant tag (partner.category)

Do the same with a move with an earlier date that will put the amounts in a different column

Before this commit, it crashed because some result variable did not contain
the key "False"

After this commit, it doesn't crash

OPW 1933749

closes odoo/odoo#30603

addons/account/report/account_aged_partner_balance.py

@@ -142,6 +142,7 @@ class ReportAgedPartnerBalance(models.AbstractModel):
 
                 if not self.env.user.company_id.currency_id.is_zero(line_amount):
                     partners_amount[partner_id] += line_amount
+                    lines.setdefault(partner_id, [])
                     lines[partner_id].append({
                         'line': line,
                         'amount': line_amount,
@@ -179,6 +180,7 @@ class ReportAgedPartnerBalance(models.AbstractModel):
                     line_amount -= partial_line.company_id.currency_id._convert(partial_line.amount, user_currency, user_company, date_from)
             if not self.env.user.company_id.currency_id.is_zero(line_amount):
                 undue_amounts[partner_id] += line_amount
+                lines.setdefault(partner_id, [])
                 lines[partner_id].append({
                     'line': line,
                     'amount': line_amount,