erp5_wendelin: Fix consistency checks on Data Bucket Stream

This fixes the following error:

  >>> data_bucket_stream = context.data_stream_module.newContent(
  >>>   portal_type = "Data Bucket Stream"
  >>> )
  >>> data_bucket_stream.insertBucket(1, 'testBucket')
  >>> data_bucket_stream.Base_getConsistencyList()

  # Error Type: AttributeError
  # Error Value: 'NoneType' object has no attribute 'getPortalType'

The returned empty lists are valid return values as it can be seen in the Folder class methods:

https://lab.nexedi.com/nexedi/erp5/blob/a17bb910/product/ERP5Type/Core/Folder.py#L1530
https://lab.nexedi.com/nexedi/erp5/blob/a17bb910/product/ERP5Type/Core/Folder.py#L1026

bt5/erp5_wendelin/DocumentTemplateItem/portal_components/document.erp5.DataBucketStream.py

@@ -139,6 +139,16 @@ class DataBucketStream(Document):
   def __len__(self):
     return len(self._tree)
 
+  # XXX: Workaround to fix errors during consistency checks.
+  # We should rename the "_tree" attribute add a migration
+  # script for existing instances. Then we can remove
+  # the following two methods.
+  def objectValues(self, *args, **kw):
+    return []
+
+  def objectIds(self, *args, **kw):
+    return []
+
   def initBucketTree(self):
     """
       Initialize the Bucket Tree
@@ -190,7 +200,6 @@ class DataBucketStream(Document):
   def _getOb(self, id, *args, **kw):
     return None
 
-    
   def getBucketByKey(self, key=None):
     """
       Get one bucket