<prepare-dataframe-and-fit-transform>
>>> train_df = pl.DataFrame({"fruits": ["apple", None, "cherry"]})
>>> encoder = sk.NullEncoder()
>>> encoder.fit_transform(train_df)
shape: (3, 1)
┌────────┐
│ fruits │
│ ---    │
│ bool   │
╞════════╡
│ false  │
│ true   │
│ false  │
└────────┘
</prepare-dataframe-and-fit-transform>
