LiveTest: set a default value for initial_transaction_hash so that _close does...

LiveTest: set a default value for initial_transaction_hash so that _close does not fail when setUp has failed

product/ERP5Type/tests/ERP5TypeLiveTestCase.py

@@ -66,6 +66,7 @@ class ERP5TypeLiveTestCase(ERP5TypeTestCaseMixin):
     - An eplicit list of exceptions to live tests remains to be
       defined.
     """
+    initial_transaction_hash = None
     portal = None
 
     def getPortalName(self):