fix(python): validate operator arithmetic with None, fix Series edge-case

[alexander-beedie]

Adds test coverage for operator arithmetic against None (and tests the associated named methods). Discovered that Series was inconsistently raising an error where the same ops on Expr and DataFrame succeeded.

Note:
This may be an implicit regression of some kind (from the 0.19.x series) as it was found via unit testing while trying to upgrade our work codebase to 0.20.x 🤔

Example

All of the following formulations work consistently...

df = pl.DataFrame({"n": [2, 3]})

df.select( pl.col("n") * pl.lit(None) )
df.select( pl.col("n") * None )
df * pl.Series([None])
df * None

# shape: (2, 1)
# ┌──────┐
# │ n    │
# │ ---  │
# │ i64  │
# ╞══════╡
# │ null │
# │ null │
# └──────┘

df["n"] * pl.Series([None])

# shape: (2,)
# Series: 'n' [i64]
# [
#   null
#   null
# ]

... only this variant failed:

df["n"] * None
# `mul` operation not supported for dtype `null`

I've fixed this and added the missing test coverage.

[stinodego]

If we want to support multiplying by None leading to a full None result, we should implement this on the Rust side and support multiplication and other operations for Null series.

I think this was 'broken' in the update where empty Series default to the Null type.

EDIT: I didn't read the description properly... maybe this is an OK fix.

[alexander-beedie]

I think this was 'broken' in the update where empty Series default to the Null type.

That worked before, and still works now ;)

EDIT: I didn't read the description properly... maybe this is an OK fix.

Indeed; operations against null already work in every single context (expr/frame/series), except for one very specific form; the fix simply makes this form equivalent to the null Series operation. The relevant Rust code already exists and works.

[stinodego]

Apologies, didn't give this the correct attention the first time around (it was late). This is a good fix!

I have one comment then this can be merged.

[stinodego]

Good to merge if CI is green 👍