perf(python): Optimize DataFrame.iter_rows for smaller buffer sizes

[alexander-beedie]

Realised we could move a check out of the inner buffer/offset loop; with the default buffer size I see negligible speedups (low single-digit percentage at best), but with smaller buffer sizes there can be a meaningful gain.

Example

(Worst case scenario; small buffer, few cols)

from codetiming import Timer
import polars as pl

df = pl.DataFrame({"x": range(500_000), "y": range(-500_000,0)})
with Timer():
    data = list(df.iter_rows(named=True, buffer_size=8))

BEFORE: 1.216 seconds
 AFTER: 0.894 seconds  (~35% faster)