code: def test_list_of_empty_lists(self):
      joiner = DocumentJoiner()
      result = joiner.run([[], []])
      assert result == {"documents": []}
  
file_path: haystack/test/components/joiners/test_document_joiner.py
imports: import logging
  import pytest
  from haystack import Document
  from haystack.components.joiners.document_joiner import DocumentJoiner
label_desc: Use image node_id 4 for calling the TestDocumentJoiner obj's underlying member method code with example usage: obj.test_list_of_empty_lists() without return types