fix: SQL COUNT(DISTINCT x) should not include NULL values

[alexander-beedie]

SQL results should not include NULL values in COUNT(colname) or COUNT(DISTINCT colname) results -- only COUNT(*) should include them (we were doing the right thing for the first form, but not the second one with the inner 'distinct').

Ref: https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-AGGREGATES

Example

import polars as pl

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

df.sql("""
  SELECT
     COUNT(x) AS count_x,
     COUNT(y) AS count_y,
     COUNT(*) AS count_star,
     COUNT(DISTINCT x) AS count_unique_x,
     COUNT(DISTINCT y) AS count_unique_y,
   FROM self  
""")
# shape: (1, 5)
# ┌─────────┬─────────┬────────────┬────────────────┬────────────────┐
# │ count_x ┆ count_y ┆ count_star ┆ count_unique_x ┆ count_unique_y │
# │ ---     ┆ ---     ┆ ---        ┆ ---            ┆ ---            │
# │ u32     ┆ u32     ┆ u32        ┆ u32            ┆ u32            │
# ╞═════════╪═════════╪════════════╪════════════════╪════════════════╡
# │ 5       ┆ 3       ┆ 5          ┆ 3              ┆ 1              │
# └─────────┴─────────┴────────────┴────────────────┴────────────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.33%. Comparing base (5fc791c) to head (da3a410).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17930      +/-   ##
==========================================
- Coverage   80.49%   80.33%   -0.16%     
==========================================
  Files        1493     1492       -1     
  Lines      196477   196330     -147     
  Branches     2813     2813              
==========================================
- Hits       158155   157726     -429     
- Misses      37801    38083     +282     
  Partials      521      521              

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