agtype: return an empty list from tail() for empty and singleton lists

[SAY-5]

Fixes #2384.

age_tail() in src/backend/utils/adt/agtype.c had an explicit early return that mapped both the empty-list and the singleton-list cases to SQL NULL:

count = AGT_ROOT_COUNT(agt_arg);
/* if we have an empty list or only one element in the list, return null */
if (count <= 1)
{
    PG_RETURN_NULL();
}

The Cypher specification — and Neo4j / Memgraph, as shown in the issue — both define tail(list) as "the list without its first element". For a zero-or-one-element input that is an empty list [], not null. Differential tests against Neo4j surface this divergence immediately.

Removing the early return is enough: the loop below already iterates from i = 1, so it naturally emits [] for count <= 1 and emits the correct tail for the general case. No behaviour change for count >= 2.

Regression tests and expected output are updated accordingly — the two "should return null" cases at the end of the tail() section now expect [] and the comment is corrected.

SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype);   -- []
SELECT * FROM cypher('list', $$ RETURN tail([])  $$) AS (tail agtype);   -- []