code: def test_no_label():
      df = pd.DataFrame({"a": [1]})
      first_line = DFtoVW(df=df, features=Feature("a")).convert_df()[0]
      assert first_line == "| a:1"
  
imports: import pytest
  import pandas
  from vowpalwabbit.dftovw import ContextualbanditLabel, DFtoVW, Feature, MulticlassLabel, MultiLabel, Namespace, SimpleLabel
label_desc: Use image node_id 1 for calling a global function with example usage: test_no_label() without return types