pl.format should be clear it will return null when one of the arguments is null

[ericvdtoorn]

Description

It was not clear to me that pl.format would make the entire column null when one of the arguments is null. i.e.

df = pl.DataFrame({
    "a": [1,2,3,4],
    "b": [5,6,None,8]
})
df.with_columns(
    ab=pl.format("{}_{}", c("a"), c("b"))
)

	a	b	ab
0	1	5	1_5
1	2	6	2_6
2	3	null	null
3	4	8	4_8

I originally expected a null to either a) throw an error or b) insert null. If this is intended behaviour, I believe that should be made explicit in the documentation to avoid tripping over this silent error.

Link

https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.format.html#polars.format