code: def test_get_indices_empty_iterator(self, schema):
      graph = StructureModel()
      # add node without parents:
      graph.add_node(10)
      mapper = VariableFeatureMapper(schema)
      x = mapper.get_indices(graph.predecessors(10))
      assert len(x) == 0
      assert isinstance(x, list)
  
imports: from collections import OrderedDict
  import pytest
  from causalnex.structure import StructureModel
  from causalnex.structure.categorical_variable_mapper import VariableFeatureMapper, validate_schema
label_desc: Use image node_id 6 for calling the TestVariableFeatureMapper obj's underlying member method code with example usage: obj.test_get_indices_empty_iterator(schema) without return types