[taskdistribution] Make node_title optional

createTestResult is called also without node_title, and it shall allow
execution in such case.

product/ERP5/Document/ERP5ProjectUnitTestDistributor.py

@@ -392,6 +392,7 @@ class ERP5ProjectUnitTestDistributor(XMLObject):
     """
     LOG('ERP5ProjectUnitTestDistributor.createTestResult', 0, (node_title, test_title))
     portal = self.getPortalObject()
+    if node_title:
       test_node = self._getTestNodeFromTitle(node_title)
       test_node.setPingDate()
     test_suite = self._getTestSuiteFromTitle(test_title)