Return an empty list from tail() for empty and singleton lists (#2399)

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) 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.

Remove the early return. 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. Tests and expected
output are updated to match the new, spec-correct result.

Author: SAY-5

regress/expected/expr.out

@@ -8039,17 +8039,17 @@ SELECT * FROM cypher('list', $$ RETURN tail(["a","b","c","d","e"]) $$) AS (tail
  ["b", "c", "d", "e"]
 (1 row)
 
--- should return null
+-- should return an empty list
 SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype);
  tail 
 ------
- 
+ []
 (1 row)
 
 SELECT * FROM cypher('list', $$ RETURN tail([]) $$) AS (tail agtype);
  tail 
 ------
- 
+ []
 (1 row)
 
 -- should throw errors

regress/sql/expr.sql

@@ -3269,7 +3269,7 @@ SELECT * from cypher('list', $$RETURN range(0, -10.0, -3.0)$$) as (range agtype)
 -- should return the last elements of the list
 SELECT * FROM cypher('list', $$ RETURN tail([1,2,3,4,5]) $$) AS (tail agtype);
 SELECT * FROM cypher('list', $$ RETURN tail(["a","b","c","d","e"]) $$) AS (tail agtype);
--- should return null
+-- should return an empty list
 SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype);
 SELECT * FROM cypher('list', $$ RETURN tail([]) $$) AS (tail agtype);
 -- should throw errors

src/backend/utils/adt/agtype.c

@@ -6056,11 +6056,11 @@ Datum age_tail(PG_FUNCTION_ARGS)
 
     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();
-    }
+    /*
+     * For an empty or singleton list, tail() returns an empty list. The loop
+     * below already produces that result (i starts at 1 so nothing is pushed
+     * when count <= 1), so we do not special-case the count here.
+     */
 
     /* clear the result structure */
     MemSet(&agis_result, 0, sizeof(agtype_in_state));