Null-partitioned grouped generators' query results overlap

[tim-band]

Null-partitioned grouped generators make one overall query plus one query per partition. Partitions have one null pattern each. The non-nulls should have a k<n> (for category columns) or m<n> (for numeric columns) in each result row, and these should not be NULL (otherwise this row should be part of a different partition as it has a different null pattern than expected). Currently src_stats.yaml can look like this:

...
auto__cov__measurement__measurement_concept_id__alt_10:
  comments:
  - Number of rows for which value_as_number IS NULL and unit_concept_id IS NULL (for
    each possible value of measurement_type_concept_id, measurement_concept_id, unit_source_value
    and value_as_concept_id)
  queries:
    date: '2025-11-21 11:26:02'
    query: SELECT 0 AS rank, _q.measurement_concept_id AS k0, _q.measurement_type_concept_id
      AS k1, _q.unit_source_value AS k2, _q.value_as_concept_id AS k3, _q.count AS
      count FROM (SELECT COUNT(*) AS count, measurement_concept_id, measurement_type_concept_id,
      unit_source_value, value_as_concept_id FROM (SELECT * FROM measurement WHERE
      unit_concept_id IS NULL AND value_as_number IS NULL ORDER BY RANDOM() LIMIT
      500) AS _sampled GROUP BY measurement_concept_id, measurement_type_concept_id,
      unit_source_value, value_as_concept_id) AS _q
  results:
  - count: 19
    k0: 0
    k1: 32856
    k2: null
    k3: null
    rank: 0
  - count: 2
    k0: 3000348
    k1: 32856
    k2: null
    k3: null
    rank: 0
  - count: 1
    k0: 3000764
    k1: 32856
    k2: null
    k3: null
    rank: 0
...
  - count: 4
    k0: 3022318
    k1: 32817
    k2: null
    k3: 21499419
    rank: 0
  - count: 73
    k0: 3022318
    k1: 32817
    k2: null
    k3: 45877096
    rank: 0
...

Note the k2: null and k3: null lines. These should not be possible.