migrate: convert missing-values.ipynb from pandas to polars

[meshackamadi]

Summary

Converts the missing-values.ipynb chapter from pandas to polars, covering explicit and implicit missing value handling.

Changes

Code

• .isna() → .is_null()
• .notna() → .is_not_null()
• .fillna() → .fill_null()
• .dropna() → .drop_nulls()
• method="ffill" → strategy="forward"
• method="bfill" → strategy="backward"
• .replace({-99: pd.NA}) → .with_columns(pl.col("*").replace(-99, None))

Text

• Updated prerequisites to mention polars
• Updated terminology: "NA" → "null"
• Updated function names to polars equivalents

Testing

• All code blocks execute successfully
• Missing value handling produces expected results
• Notebook renders without errors

Files Modified

• missing-values.ipynb