polars diff with leading zero

diff0.py

def diff0(x: pl.Expr, first_value=0.0) -> pl.Expr:
    """
    Get the differenced vector, putting a certain value (not null)
    in the first place
    """
    i = pl.arange(0, x.len())
    return pl.when(i == 0).then(first_value).otherwise(x.diff())