code: def test_concatenate_columns_null_values(missingdata_df):
      """Test for concatenating columns with null values.
  
      Null values ought to show up as "nan" in strings
      in the concatenated column.
      """
      df = missingdata_df.concatenate_columns(
          column_names=["a", "decorated-elephant"],
          sep="-",
          new_column_name="index",
          ignore_empty=True,
      )
      expected_values = ["1.0-1", "2.0-2", "nan-3"] * 3
      assert expected_values == df["index"].tolist()
  
imports: import pytest
  from janitor.errors import JanitorError
label_desc: Use image node_id 2 for calling a global function with example usage: test_concatenate_columns_null_values(missingdata_df) without return types