fix: Preserve NULLs for is_not_nan

[nameexhaustion]

Fixes #15862.

As an aside, float-specific functions on non-floats also currently don't preserve NULLs:

Non-float

import polars as pl

df = pl.Series("a", [1, None]).to_frame()

v = df.with_columns(
    **{
        x: getattr(pl.col("a"), x)().alias(x)
        for x in ["is_nan", "is_not_nan", "is_finite", "is_infinite"]
    }
)

print(v)

shape: (2, 5)
┌──────┬────────┬────────────┬───────────┬─────────────┐
│ a    ┆ is_nan ┆ is_not_nan ┆ is_finite ┆ is_infinite │
│ ---  ┆ ---    ┆ ---        ┆ ---       ┆ ---         │
│ i64  ┆ bool   ┆ bool       ┆ bool      ┆ bool        │
╞══════╪════════╪════════════╪═══════════╪═════════════╡
│ 1    ┆ false  ┆ true       ┆ true      ┆ false       │
│ null ┆ false  ┆ true       ┆ true      ┆ false       │
└──────┴────────┴────────────┴───────────┴─────────────┘

[codspeed-hq]

CodSpeed Performance Report

Merging #15889 will not alter performance

_{Comparing nameexhaustion:nan (89fa1dd) with main (66c9aa4)}

Summary

✅ 13 untouched benchmarks

[ritchie46]

Thank you @nameexhaustion!

[ritchie46]

I think we can save a negate + allocation if we say & ca.is_not_null(). In that case we can use the validity mask directly and convert that to boolean array.

[nameexhaustion]

not sure what's going on with code coverage but I'll give it one more try

libunwind: stepWithCompactEncoding - invalid compact unwind encoding

warning: /Users/runner/work/polars/polars/target/polars-41273-14359842420759467167_2.profraw: invalid instrumentation profile data (file header is corrupt)
error: no profile can be merged
error: failed to merge profile data: process didn't exit successfully: `/Users/runner/.rustup/toolchains/nightly-[20](https://github.com/pola-rs/polars/actions/runs/8841603756/job/24278957655?pr=15889#step:13:21)24-04-15-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/bin/llvm-profdata merge -sparse -f /Users/runner/work/polars/polars/target/polars-profraw-list -o /Users/runner/work/polars/polars/target/polars.profdata` (exit status: 1)

[ritchie46]

libunwind: stepWithCompactEncoding - invalid compact unwind encoding

Yeah, it is broken on every PR. :/