feat: Add SQL support for NULLS FIRST/LAST ordering

[alexander-beedie]

Builds on #16639, integrating the new capability (per-column nulls first/last) as SQL.

Examples

import polars as pl

df = pl.DataFrame({
    "x": [2, 3, None, None],
    "y": [3, 2, None, 1],
})

df.sql("""
  SELECT * FROM self 
  ORDER BY x NULLS FIRST, y DESC NULLS LAST
""")
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ null ┆ 1    │
# │ null ┆ null │
# │ 2    ┆ 3    │
# │ 3    ┆ 2    │
# └──────┴──────┘

df.sql("""
  SELECT * FROM self
  ORDER BY x DESC NULLS LAST, y NULLS FIRST
""")
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ 3    ┆ 2    │
# │ 2    ┆ 3    │
# │ null ┆ null │
# │ null ┆ 1    │
# └──────┴──────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.43%. Comparing base (a3d65df) to head (1ba22f1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16711      +/-   ##
==========================================
- Coverage   81.45%   81.43%   -0.02%     
==========================================
  Files        1413     1413              
  Lines      186187   186188       +1     
  Branches     2776     2776              
==========================================
- Hits       151655   151623      -32     
- Misses      34012    34045      +33     
  Partials      520      520              

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[MarcoGorelli]

This isn't code I'd looked at before, but the logic looks very simple (and, as far as I can tell, correct)

Nitpick but it looks like now def foods_ipc_path happens in two places? Not an issue (and personally I'd prefer it this way, rather than having it magically happen in a conftest.py), just pointing it out

[codspeed-hq]

CodSpeed Performance Report

Merging #16711 will not alter performance

_{Comparing alexander-beedie:sql-nulls-first-last (1ba22f1) with main (a3d65df)}

Summary

✅ 37 untouched benchmarks

[alexander-beedie]

Updated/rebased (yet again :))