Propagate null through unnest and single() three-valued logic

[MuhammadTahaNaveed]

Two related defects in how AGE handled cypher null inside list-iterating constructs.

age_unnest packaged every iterated element as a non-SQL-NULL agtype datum, even AGTV_NULL scalars. SQL IS NULL / IS NOT NULL then couldn't see those nulls, so [x IN [null, 1] WHERE x IS NULL] dropped the null it was meant to keep, and WHERE x IS NOT NULL kept the null it was meant to drop. The same mismatch surfaced in UNWIND. AGE already treats SQL NULL as the row-level representation of cypher null elsewhere (RETURN null AS v yields SQL NULL, strict operators short-circuit on it); age_unnest now does the same by emitting the row with nulls[0] = true when the element is AGTV_NULL.

single() previously transformed to SELECT count(*) FROM unnest(list) AS x WHERE pred IS TRUE, with the grammar wrapping the result as (subquery) = 1. With the unnest fix, [null, 5] WHERE x > 0 left one definite true after the WHERE filter -> count = 1 -> true. Neo4j returns null because the unknown predicate could itself be a second match. Rewritten to a CASE built on count(*) FILTER (WHERE pred IS TRUE) and bool_or(pred IS NULL):

CASE WHEN count() FILTER (WHERE pred IS TRUE) >= 2 THEN false
WHEN bool_or(pred IS NULL) THEN NULL
WHEN count() FILTER (WHERE pred IS TRUE) = 1 THEN true
ELSE false END

The >=2 arm runs first so two definite trues dominate any unknowns. Fits inside the existing make_predicate_case_expr helper alongside all/any/none, removes the special-case transform branch and the grammar = 1 wrap. A small make_count_star_filter_agg helper mirrors the existing make_bool_or_agg. Verified against Neo4j for the new edge cases (one-true-plus-null, two-trues-plus-null, all-nulls, mixed-true-false-null).

The predicate_functions regression also picks up the corrected behavior of any/all/none over null elements: null > 0 now yields SQL NULL instead of being silently treated as true, so the three-valued combinators in those functions produce the openCypher results the comments previously documented as buggy.