Do not compute account party for general ledger line without party

Closes #12570
(grafted from d7fee1d3bf9107875d7ac7bd447b1e6e563c33aa)

--HG--
branch : 6.0

modules/account/account.py

@@ -2236,6 +2236,8 @@ class GeneralLedgerLine(ModelSQL, ModelView):
         account_ids, party_ids = set(), set()
         for r in records:
             account_parties[r.id] = None
+            if not r.party:
+                continue
             account_party2ids[r.account.id, r.party.id].append(r.id)
             account_ids.add(r.account.id)
             party_ids.add(r.party.id)