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