Use transaction date as origin date when value date is empty

issue11292
review366831002
(grafted from 3e51b8608797844e8fc318ceac7ae5440cd0b7fc)

--HG--
branch : 6.0

statement.py

@@ -77,7 +77,10 @@ class ImportStatement(metaclass=PoolMeta):
         pool = Pool()
         Origin = pool.get('account.statement.origin')
         origin = Origin()
-        origin.date = transaction.valueDate
+        try:
+            origin.date = transaction.valueDate
+        except ValueError:
+            origin.date = transaction.transactionDate
         origin.amount = transaction.amount
         descriptions = []
         for item in transaction.optionalItems: