code: def test_feature_with_nan():
      df = pd.DataFrame(
          {"y": [-1, 1, 1], "x1": [1, 2, None], "x2": [3, None, 2]}
      )
      conv = DFtoVW(
          df=df,
          features=[Feature("x1"), Feature("x2")],
          label=SimpleLabel("y"),
      )
      lines = conv.convert_df()
      assert lines == [
          "-1 | x1:1.0 x2:3.0",
          "1 | x1:2.0 ",
          "1 |  x2:2.0",
      ]
  
imports: import pytest
  import pandas
  from vowpalwabbit.dftovw import ContextualbanditLabel, DFtoVW, Feature, MulticlassLabel, MultiLabel, Namespace, SimpleLabel
label_desc: Use image node_id 12 for calling a global function with example usage: test_feature_with_nan() without return types