ProcessingNodeTestCase: Adjust _p_resolveConflict for more than 2 nodes

Usually we run tests with two nodes, so when there's a conflict it's
always between the "main" node and the processing node, so we always
have a distributing node during conflict.

But when running with, for example, 2 nodes (the main node + 2 nodes),
the conflict might occur between the two processing nodes and after
the resolution we still don't have a distributing node.

product/ERP5Type/tests/ProcessingNodeTestCase.py

@@ -108,9 +108,11 @@ def Application_resolveConflict(self, old_state, saved_state, new_state):
       saved_state.pop('test_distributing_node', ''),
       new_state.pop('test_distributing_node', '')}
   test_distributing_node_set.discard('')
-  if len(test_distributing_node_set) != 1:
-    raise ConflictError
-  new_state['test_distributing_node'] = test_distributing_node_set.pop()
+  new_state['test_distributing_node'] = ''
+  if test_distributing_node_set:
+    if len(test_distributing_node_set) != 1:
+      raise ConflictError
+    new_state['test_distributing_node'] = test_distributing_node_set.pop()
 
   old, saved, new = [set(state.pop('test_processing_nodes', {}).items())
                      for state in (old_state, saved_state, new_state)]