[FIX] account: aged partner balance report consistent return

Before this commit, the function that computes the aged partner balance
could return a list instead of a dict if no partner were found

After this commit, we make the function's signature consistent
closes #26095

addons/account/report/account_aged_partner_balance.py

@@ -68,7 +68,7 @@ class ReportAgedPartnerBalance(models.AbstractModel):
         partner_ids = [partner['partner_id'] for partner in partners if partner['partner_id']]
         lines = dict((partner['partner_id'] or False, []) for partner in partners)
         if not partner_ids:
-            return [], [], []
+            return [], [], {}
 
         # This dictionary will store the not due amount of all partners
         undue_amounts = {}