Do not fail to set default fiscal year when context value is None

We can not compare contextual fiscalyear or period as positive if their values
have been set to None.

Closes #12502

modules/account/account.py

@@ -2179,10 +2179,12 @@ class GeneralLedgerAccountContext(ModelView):
         FiscalYear = pool.get('account.fiscalyear')
         Period = pool.get('account.period')
         context = Transaction().context
-        if context.get('fiscalyear', -1) >= 0:
-            return context['fiscalyear']
-        elif context.get('period', -1) >= 0:
-            period = Period(context['period'])
+        fiscalyear_id = context.get('fiscalyear')
+        period_id = context.get('period')
+        if fiscalyear_id is not None and fiscalyear_id >= 0:
+            return fiscalyear_id
+        elif period_id is not None and period_id >= 0:
+            period = Period(period_id)
             return period.fiscalyear.id
         else:
             try: